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A small electronic device for purifying water with a mixed oxidizer based on electrolyzing salt.

Overall Rating: Below Average

This is a concensus rating from all the reviewers. It works as specified, provided you get the batteries inserted correctly, but the chlorine taste was unacceptable to most reviewers and the waiting time can be extremely long.

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by Roger Caffin, with contributions from Backpacking Light staff members Doug Johnson, Mike Martin and Will Rietveld |



There is a continuing debate in the outdoors world as to whether you need to purify all the water you drink. Some people use expensive filters, some use chemicals, some use rather simplistic gravity filters of unproven merit, while some people just don't bother. One thing is clear from this: if you are going to use something, it should do what it claims (and this is not always true), be reasonably light, easy to use, work fairly fast and produce water with an acceptable taste.

The name MIOX stands for MIxed OXidants, which is what the MIOX unit produces when it electrolyses a concentrated salt solution. The chemicals generated include 'a potent cocktail of chloroxygen compounds such as hypochlorous acid and chlorine', to quote MSR. To do this it runs on batteries. However, you should note that the hypochlorous acid is diluted by hypochlorite, which is a weaker chemical.

The author's initial assessment was that the water produced smelt and tasted strongly of chlorine. In view of this it was decided that a round trial should be conducted with four members of the Backpacking Light staff (the author, Doug Johnson, Will Rietveld and Mike Martin) to assess ease of use and the taste of the resulting water. Two wives were also included as guinea pigs.

What's Good

  • Very light compared to a pump filter
  • Very easy to carry
  • Easy to use once the batteries are properly inserted

What's Not So Good

  • Fairly strong chlorine taste and smell
  • Relies on batteries, which can run out
  • Up to 4 hour waiting period for some bugs (not unique to this product however)
  • Inserting batteries can be tricky, with several failures
  • Needs consumable test strips to tell whether enough oxidant has been generated
  • Rather expensive (compared to buying any of the chlorine dioxide liquids for instance)



MSR Corporation

  Year of Manufacture

2006 (presumed)

  Treatment times

Viruses, bacteria: 15 min
Giardia: 30 min
Cryptosporidium: 4 hr


Pen: 99 g (3.5 oz)
Kit: 227 g (8 oz)

  Kit contents

MIOX Purifier, salt, batteries, safety-indicator strips, instruction booklet, quick-reference card, and storage sack




The world of water treatment and purification is characterized by vast amounts of mis-information and hype. Almost every vendor will certainly warn you about the immense, almost unbelievable, hazards you face when you leave behind the chemically treated and filtered municipal water supplies. In the light of these warnings, it is curious that so many walkers don't bother treating their water, and don't get sick. The principal author does not always treat his drinking water either.

MIOX kit
The field-use kit


There are many devices on the market which do little more than remove the large bits of floating matter you can find in water. Typically they are gravity-fed from some sort of bladder. Equally typical is the fact that they don't have any sort of EPA certification. To the best of my knowledge, many of them don't stop either viruses or bacteria, although some exceptions exist. Caveat Emptor.

There are good filters on the market (often called microfilters), but few of them filter to finer than 0.3 microns, and viruses are all much smaller than this. You should note that the term 'microfilter' has no legal meaning. So using a filter leaves you exposed to all of the viruses - and this can be especially hazardous when traveling in Third World countries, or when near a town sewerage treatment plant. Of course filters can block up as well, leaving you wondering what to do next. I have used the Katadyn Hiker filter for many years.

There are several different classes of chemicals available (e.g. hypochlorite solutions, pentavalent iodine tablets, chlorine dioxide solutions), but each of them has its own disadvantages. Some chemicals just don't work the way we need, some have little taste but don't last very long before they decay; others are longer lasting but have an awful taste. Some simply don't work very well or take forever to have an effect. Anyhow, while most chemicals can inactivate viruses fairly quickly, in general they take hours to work on protozoa like Giardia lamblia and Cryptosporidium species.

The MIOX can be compared with other treatments like Aquamira and Polar Pure, as was done in 'Efficacy of Chemical Water Treatment Technologies in the Backcountry' by Erica McKenzie and Dr. Ryan Jordan, Backpacking Light print magazine, Spring 2005, pp24+. In that article the authors state that the output of the MIOX is 'a chlorine species, and exists as a combination of hypochlorous acid and hypochlorite. As previously stated, hypochlorite is considered a weaker disinfectant than hypochlorous acid. Therefore, given the same concentration of oxidant as free chlorine, one would expect Aquamira to be more efficacious than MIOX.'

Controls and LEDs on the MIOX unit

Finally there are now Ultra Violet light treatment systems coming on the market. These work fine on viruses, bacteria and protozoa and leave no taste at all, but at present they are a little hungry on batteries. That may change in a year or two when UV-emitting LEDs become cheaper. I currently use a Steripen Adventurer when I need to treat any water.

Design and Construction

The MIOX is really very simple. It is a red tube, with an electrolysis cell at one end and a battery case at the other end. The small cell has two electrodes and in use contains a concentrated salt solution. Current is driven through the salt solution to convert the water and chloride ions from the salt into hypochlorous acid and several other chlorine compounds - most of which have some oxidizing power. It is this oxidising power which kills the bugs. I will spare you the rest of the chemistry, which is explained on the MSR web site anyhow. The rest of the MIOX unit is the battery case and the electronics which controls the current through the cell.

The whole thing is controlled by a microprocessor - and you thought life in the wilderness was meant to be simple? There is a GO button - the grey dot in the photo to the right. If the Battery Low light comes on, you know the batteries are exhausted. If the Salt Low light comes on, you need to add more salt - I assume that this is monitored by conductivity through the salt solution. Finally there is a Run light to say it is working.


This is one case where 'first read the manual' really applies. I am going to quote from the MSR web site for most of the instructions as they have phrased it better than I can.

"Battery installation: Unscrew the black battery cap from the bottom of the purifier. Lift the round end of the flexible battery contact strip and insert the batteries into the battery chamber. You may have to use a blunt object such as a pen to gently extend the flex strip. See the picture on the purifier body depicting the orientation of the batteries. Fold the strip over so that the end of the last battery and round end of the strip make contact. Then screw the battery cap firmly to engage the O-ring seal. [Some testers had trouble with this step.]

The two caps on the operational end of the MIOX

Salt installation: Unscrew the salt chamber cap, which is the little black cap at the very top. If you unscrew the entire salt chamber instead you will see the end of a metal post surrounded by a small void and then a metal tube. If this is the case, look for the small cap at the top of the salt chamber and unscrew it. Next, fill the salt chamber about 2/3 full with salt. [MSR provides some rock salt with the kit.] Some room at the top will allow good water flow. The salt needs to be moistened before use if it is dry. Add a few drops of water to the salt chamber to wet the salt.

Use: Fill a container with the water to be treated and note the volume. Unscrew the black salt chamber from the colored MIOX Purifier's body. You will see the end of a metal post surrounded by a small void and then a metal tube - the electrolytic cell. Submerse the purifier or pour a few drops of the untreated fresh water from the water source to fill the electrolytic cell, the void between the metal post and tube. Cap and shake the purifier 10 times to mix the water and the salt in the salt chamber. (You may need to shake more than 10 times depending on the water source and type of salt used.) This creates a brine solution. Look at the chart on the purifier to determine the number of button clicks required for the amount of water you wish to purify. For example, two clicks will make enough disinfectant to treat one liter. Hold the purifier upright and remove the cap to expose the electrolytic cell filled with the brine solution. Next, activate the purifier by pressing the gray button. [At this stage, all being well, there is a lot of fizzing from the cell for a while.] Mix the contents of the cell with the water to be treated and follow the instructions for using the safety-indicator strips to determine if the concentration is high enough for effective disinfection. After verifying the concentration, wait the appropriate dwell time before drinking."

Waiting Time - a Problem

The last line in the instructions above is the one which gives many of us a problem: 'wait the appropriate time'. Waiting 15 minutes for viruses and bacteria to be killed is a bit slow compared to other methods of treatment, but having to wait for 4 hours for Crypto to be killed is just a bit too long. Part of the problem here is that you probably won't know what species of bugs you must cater for each time, so you end up having to allow 4 hours every time. In hot weather in dry country, this can be a very long time.

Sure, you can treat your water in the evening and let it 'cook' overnight - but then you will have to carry that water on the next day. You can't just treat another litre suddenly for a quick drink. In real-life practical terms, what the 30 minutes for Giardia and 4 hours for Crypto means is that you are going to have to carry a litre (a quart) of water around with you most of the time so you can have something to drink while waiting for the next batch to be safe. For many people this will not be very satisfactory.

Field Testing - Author

After I tested the unit I sent it to three other Backpacking Light Senior Editors, to get their opinions about taste and ease of use. We will start with my test results, then go on to the comments from the others.

First I carefully read the instructions. Following these, I added the batteries and the salt to the unit. Then I added a little water to wet down the salt, and then I filled up the cell with water. Finally I activated the unit by pressing the grey button for 1 litre of water. It ran for a moment, then both the salt and the battery LEDs flashed red. This puzzled me as the batteries were new.

So after reading the instructions again, I decided that maybe I had not wet the salt crystals down enough. I gave the unit another 50 shakes - the instructions do mention that this may be necessary. Then I tried reactivating it. Obviously my shaking had worked this time: the unit immediately started to fizz - quite vigorously. After a short time (it felt like just a few seconds) the Run light went out and the fizzing stopped. Hum - that was fast, but did it work? I poured the contents of the cell into my 1 litre bottle and shook it up to mix it. The water I was using was rain-water, not chlorinated town water.

MIOX Test Strips
Matching the Test Strip colours

Now I could see why MSR include the test strips: how do you know whether anything useful has been done? I inserted a test strip and was rewarded with an instant colour change, to purple. I compared the colour of the strip with the colours on the side of the small canister which holds the rest of the strips. I have to say that the printing of those colours is not very good: it was hard to tell where the colour I had would fit into the range shown. I think the wet surface of the test strip may have contributed to the difficulty. Anyhow, the colour seemed to lie between 'OK' and 'OK+', which matched the fact that I had given the unit about 1 and a half treatment cycles.

Then I waited 10 minutes and retested the water with another strip. The colour change was slower, but it still turned purple - meaning 'OK'. Doing this testing regularly could consume a lot of test strips of course, unless you became sufficiently confident to skip their use after a while. MSR does sell extra test strips of course.

I then closed the bottle and waited another 10 minutes before sampling the water. I was not impressed by the results: the water both smelt and tasted fairly strongly of 'swimming pool chlorine'. The smell was a little bit stronger than that which I am used to from Coghlans iodine tablets, which while present is usually quite tolerable. On the other hand, the Coghlans iodine tablets rarely leave any taste in my experience. I have to say I would not really want to have to rely on the MIOX for nice fresh drinking water in the field.

Since I always go walking with my wife, I thought it prudent to ask her what she thought of the water. After a sniff and a taste she told me quite definitely (you may interpret that as you wish) that she would not be drinking that water in the field. She is usually happy to drink the iodine-treated water.

In the Backpacking Light article cited above the authors state that 'MIOX has more than six-fold the oxidant concentration of either of the other systems, and requires the greatest treatment time.' Apparently the process converts some of the chlorine ions from the salt to the less effective hypochlorite ions rather than to hypochlorous acid, and this is why the much higher overall concentration is needed: it's to get the same bug-killing power. The increased concentration of chlorine species may well account for the very noticeable taste and smell.

Field Testing by other Backpacking Light Staff Members

After I had run the unit through several cycles, I passed it on to Doug, Will and Mike to get their impressions. Doug thought the batteries were dead when he received the unit in the mail from me, and had to replace them. They could perhaps have drained in the fairly short time it took the Post Office to get the parcel from Australia to America by air mail, or he may have had battery contact problems at first. I doubt the former as I thought I had removed the batteries from the unit before posting. We are not sure what the problem was, but see Will's report about this as well. Mike had less trouble with the unit.


Q: Can you smell anything at first?
A: YES - especially when I made a more concentrated mix but still with the 1L mix.

Q: Can you taste anything at first?
A: Absolutely yes, but it isn't horrible. It's hard to describe - a little salty maybe - like drinking very hard water that's high in minerals. It's not horrible but definitely distinctive. Better than Iodine water but stronger than chlorine dioxide.
Note: Doug's wife felt that it would be drinkable in the field but not at home.

Q: Can you smell anything after a few hours?
A: It was certainly diminished but still there.

Q: Can you taste anything after a few hours?
A: The taste was also diminished to some degree but still present. My wife and I could both instantly tell the MIOX water from regular water.

Q: Would you want to use this as your primary water treatment method in the field?
A: No way. Too reliant on batteries, too heavy, too complicated in use, too weird in the taste department. I'll stick to a [different brand] chlorine dioxide treatment - simple, effective, lightweight, and less taste/smell issues.


I assembled it, and charged it according to the directions and it didn't work at first. I spent a lot of time testing the batteries and re-inserting them and could not get it to work. So I contacted MSR and told them of the problem. I tried it again the next day before sending it in, and it worked. I don't know exactly what the problem was.
[The suspicion is that Will had problems with the small flap which folds over the battery. This may be what Doug had problems with too. RNC]

I used it to treat 1 liter of distilled water (2 button presses), mixed it, and tested it with the test strip, which turned purple right away. The treated water has a distinct chlorine taste to it. I'm not offended by the taste, but it probably would interfere with the taste of tea or coffee.

The system is a bit klutsy in my opinion. There are a number of steps to the process, so it is no easier than Aqua Mira or Klearwater. The new UV water purifiers seem to be simpler and about the same weight, assuming they work. [See the author's review of the Steripen Adventurer for more information about a UV device.]


I found the MIOX purifier to be a relatively heavy, complex, battery-dependant electronic device. I really could see no advantage over simpler, lighter and les expensive chemical treatments, so I would not personally choose to use it in the field. The dosing method seems inherently imprecise, though perhaps the microprocessor compensates somehow for variables like salinity and battery life. The included test strips, while consumable and adding further weight and complexity, at least provide visual confirmation of appropriate dosage.

I tested the device using Coeur d'Alene Lake water. Immediately after treatment I noticed a slight chlorine smell to the water. The smell appeared to grow stronger over a 4 hour period, presumably due to outgassing from the solution into the airspace in the sealed bottle. Despite the smell, the water tasted fine to me - comparable to other chemical treatments.

What's Unique

The only electronic unit on the market

Low weight compared to a pump filter

Recommendations for Improvement

None really, given the limitations of the basic chemistry


"MSR MIOX Purifier REVIEW," by Roger Caffin, with contributions from Backpacking Light staff members Doug Johnson, Mike Martin and Will Rietveld. (ISSN 1537-0364)., 2007-12-05 01:30:00-07.


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Dominic Reitman
(dreitman) - F
My MIOX experience on 03/11/2008 20:50:35 MDT Print View

I received my MIOX purifier as a birthday gift about three years ago and have used it almost exclusively for all my backpacking trips. Here are my comments on the device

I'll agree that the taste is sometimes not the best but it does not bother me. I usually drink through a blatter which I noticed is better than dinking from a nalgene bottle.

After going through the original pack of test strips I no longer carry them. I also bought a lifetime supply of rock salt from the grocery store (Morton Ice Cream Salt $2). This leaves batteries as the only ongoing cost.

I have had to replace the batteries twice in three years. The batteries are not cheap, I usualy buy Energizer Lithium size 123. One set of batteries can easily last a weeklong outing for a small group of hikers (4 to 6 people). Note that you can usually get a few days of usage even after receiving the low battery signal.

As far as what to take when hiking:
Extra set of batteries and salt. Even though you can fill the salt chamber and purify what seems to be and endless amount of water... time does kill all... and after a day or so the residual moisture will desolve a lot of the salt.

My MIOX was very popular on a boyscout backpacking trip last summer. It purified water for 14 people for three days.

Dan Durston
(dandydan) - F

MIOX on 07/26/2009 12:33:46 MDT Print View

I've had a Miox for over 2 years now and I've generally been happy with it.

The battery life is excellent. As long as my batteries are relatively new, I have no worries about going on a weeklong trip with 2-3 people with it. I leave the test strips at home and just bring the actual gadget plus a small bag of salt. You certainly don't need nearly the whole baggie of salt that comes with the Miox.

I haven't had any troubles with inserting the batteries like BPL did.

The taste is acceptable in my opinion and my wife's. I find you can smell it pretty clearly, but there isn't much actual taste. If you do find taste to be a problem you can leave the lid off your water container for 30 min once the treatment period is over to let the chlorine evaporate off.

I don't wait longer than 30 minutes to drink mine. I'm not sure why BPL felt the need to wait 4 hours, since most other treatment methods don't even kill crypto in the first place. Just forget about the crypto and wait 30 minutes. Also, 4 hours is the time at which you have a 99.99% (or something like this) chance of killing all crypto. After 30 minutes you've probably got a 98% chance that any crypto is dead. Considering how rare crypto is, those are pretty darn good odds even after 30 min.

What's not to like? It can be a bit finicky to get enough salt in the water. If you don't shake it enough, you get the low salt light. If you remember to pre-wet the salt and also don't overfill the salt chamber then it's easier to use.

Also, the whole unit is a bit heavy compared to the drops and pills on the market. I'm at work so I don't have the weight spec on mine, but it's a lot heavier than carrying a few droppers of chemicals (ie. repackaged Aquamira).

If I do stop using the Miox one day, it will be because of the weight. The smell/taste and potential for unit failure are not significant concerns for me. I find that most other people do not complain about the taste. When I've been camping with large groups, people often ask me to treat their water too and everyone loves it. The Miox is really cool to see working.

Two weeks ago my wife and I were camping with another couple that were using a microfilter pump. Initially they said they preferred pumping their water for the fresh spring water taste, but by the second day in bug infested conditions they were much happier to use the Miox and not have to spend 5 minutes down at the stream pumping water. When time is tight, weather conditions are adverse or the bugs are really bad then the Miox shines. I carry mine in my hip belt pocket and I can fill up my Platypus 1L, treat it and be on my way in well under a minute.

The reason I'm writing this review is because last night I mentioned to my wife I might sell the MIOX and use drops instead to save weight. She was really surprised and asked me to keep the MIOX because it is her favorite piece of gear. I had no is pretty darn cool...and fast....and safe.

I think I'll keep the Miox until someone comes out with drops that work as well the Miox without the 5 minute waiting period that Aquamira has. Waiting sucks when it's pouring rain.

Now that I think about it, how many water stops do you do in a full day of hiking? 4 or 5? If the Miox saves you 4 minutes per stop (1 min vs. 5 min) then that's 16-20 minutes per day. Shaving a few ounces off your pack isn't going to make that back so the MIOX hiker should be faster than the Aquamira hiker.

I weighed my MIOX to see how it all adds up. Here's the results:

99.8g - MIOX wand loaded with batteries and salt (all you need for 1-2 nights)
62.5g - MIOX wand without batteries and salt
16.5g - One CR123 Lithium Battery. The MIOX uses 2.
31.7g - MSR Salt Bag filled with Salt. This is a LOT.
10g - The amount of salt I'd bring for a 2 person 7 day trip
13g - My test strip container about 1/3 full of strips. A full container is likely around 20g.
15g - Supplied Bag

For trips that are 1 or 2 nights, I would just bring the wand loaded with fresh batteries and salt (99.8g). For trips up to a week, I would bring about 10g of extra salt. For trips longer than a week I would bring 2 spare batteries (33g) and more salt. So a MIOX setup would weigh between 100g - 150g depending on weather you are going for a few nights or a few weeks. If you wanted to bring the bag and test strips you'd be adding another 50g or so.

Edited by dandydan on 07/31/2009 20:13:39 MDT.

Ben 2 World
(ben2world) - MLife

Locale: So Cal
Re: MIOX on 08/01/2009 00:57:58 MDT Print View

Without negating the above, my big problem with MIOX -- as compared to Steripen or similar UV devices -- is that if I feel I need to treat my water at all, then I would rather spend 45-90 seconds treating my water and knowing that it is safe to drink -- versus treating it with MIOX and then having to wait anywhere from 30 minutes to 4 hours! for the chemicals to kill protozoa (e.g. crypto, giardia, etc.).

Dan Durston
(dandydan) - F

MIOX vs. Steripen Adventurer on 08/02/2009 01:08:05 MDT Print View

The Steripen is an interesting competitor. It takes marginally longer to do, but then you can drink right away and then is no chemical smell or taste. The price and weight are very similar. On the face of it, that sounds like a compelling case.

There are a few areas where the MIOX would shine though. One is battery life. The Steripen is rated at 50L per set of CR123 batteries whereas the MIOX is rated over 200L on the same set of CR123. This means lower operating costs in the long term (not really a big deal) and it means more weight on some trips. I wouldn't take spare batteries on a week long trip with 2 people for the MIOX (or even 2 weeks), but you'd probably want them for the Steripen. Since the MIOX uses about 1/4 the batteries, this could be significant on a longer trip or a trip with lots of people.

Secondly, the MIOX can crank out a lot more water. The Steripen works good for individual use, but it would make water stops long if you have 4 people who all need to make a litre or two of water. If everyone brings their own, that would obviously add a lot of weight. The MIOX can crank out a batch of MIOX in about 10-20 seconds and that batch can do up to 4 litres of water.

Thirdly, I believe the MIOX works better with turbid water but I'm unsure about this.

For solo hiking the Steripen is probably the way to go since you aren't making much water and battery life likely won't be an issue. For longer trips or larger groups the MIOX becomes increasingly advantageous.

Edited by dandydan on 08/02/2009 01:09:18 MDT.

Ben 2 World
(ben2world) - MLife

Locale: So Cal
Re: MIOX vs. Steripen Adventurer on 08/02/2009 06:49:28 MDT Print View


I totally agree -- in terms of treating water for a large group -- it would be seriously tedious to treat with Steripen. I would use something else -- but STILL NOT MIOX.

Again, convenience is one thing, but effectiveness is the critical factor here. My big hang up is the 30 minutes to 4 hours wait time to treat protozoa -- which can have both long and serious effects! Overnight treatment aside, waiting up to 4 hours for water (say in the middle of the day) is just too impractical. Compared to this major shortcoming -- the benefits of longer battery usage, etc. are tiny as to be almost meaningless.

Not saying that Steripen is "the" answer (it may or may not be depending on circumstances) -- but saying that MIOX falls short in terms of high initial cost, slightly tedious usage (as compared to chlorine dioxide tablets for example) -- but most of all for the overly long treatment time required to protect against protozoa.

Edited by ben2world on 08/02/2009 06:52:44 MDT.

Dan Durston
(dandydan) - F

Crypto? on 08/02/2009 11:28:07 MDT Print View

Does Steripen kill crypto?

My understanding is that crypto is so rare in North American waters that it's not worth worrying about. I could be wrong though. I normally wait only 30 min, which is a lot longer than Steripen but it's not that annoying.

Rick Dreher
(halfturbo) - MLife

Locale: Northernish California
Re: Crypto? on 08/02/2009 11:35:31 MDT Print View

My understanding is UVC inactivates both giardia and crypto cysts, of course with no time lag like chemical treatment.