What chlorine dioxide is, and how it differs from chlorine
The two share a word and very little else. One is a gas dissolved in water that works by taking electrons; the other is what is in household bleach. The difference decides what each is good for.
What it is
Chlorine dioxide is a compound of one chlorine atom and two oxygen atoms, ClO₂, and at room temperature it is a gas. In use it is a gas dissolved in water rather than a salt dissolved in water — it stays as ClO₂ molecules instead of breaking apart, and that is the whole of why it behaves differently from bleach.
It disinfects by oxidation: it takes electrons from the cell walls and proteins of what it meets, and they fail. Chlorine, by contrast, substitutes itself into the organic matter it finds. Same goal, different mechanism, and the by-products are not the same.
One thing to be clear about
The same name is attached to products sold as MMS or “Miracle Mineral Solution”, promoted to people as a cure for illness. They are concentrated chlorine dioxide solutions, the FDA has warned against drinking them, and they have caused real harm.
Nothing on this site is sold for that. Chlorine dioxide is a water treatment chemical and a disinfectant, used at the dilutions on the label. We make no health claim of any kind, and any supplier who does should be the end of the conversation.
Against chlorine, line by line
Where the chemistry differs. Which one to actually use for a given job is its own page.
- What it does
- Chlorine dioxide — Oxidises. Takes electrons from what it destroys.
- Chlorine — Chlorinates. Substitutes chlorine into what it meets.
- By-products
- Chlorine dioxide — Does not form trihalomethanes with organic matter.
- Chlorine — Reacts with organics to form them.
- pH
- Chlorine dioxide — Works across a wide range, including alkaline water.
- Chlorine — Loses much of its effect as water turns alkaline.
- Taste and smell
- Chlorine dioxide — No chlorophenol 'swimming pool' note.
- Chlorine — Produces it with phenols in the water.
- How you get it
- Chlorine dioxide — Made where it is used. Too unstable to ship at strength.
- Chlorine — Shipped as bleach or tablets, ready to use.
Why it is never sold ready-made
Chlorine dioxide in solution does not keep. It comes apart in light and warmth, and a bottle of it at working strength would be worth less every week it sat on a shelf. So it is generated where it is used: a stabilised precursor in one bottle, an activator in the other, mixed a few drops at a time and left to react for a few minutes.
The amber colour that appears while it stands is the chlorine dioxide itself. That is the reaction finishing, and it is how you know the mixture is ready. See how the kits are made up →
Where people actually use it
At municipal scale it has treated drinking water since the 1940s, and it is used in food and beverage processing and in pulp bleaching. At the scale most people meet it, it disinfects water storage: an RV or marine tank, a well, water put by for an emergency.
For keeping stored water drinkable over months rather than disinfecting it once, the usual choice is a stabilised preserver instead — the water treatment line covers that, and how much to use works out the dose.
Questions we are asked
- Is chlorine dioxide the same as chlorine or bleach?
- No. Chlorine dioxide is a dissolved gas, ClO₂, and it disinfects by oxidation. Chlorine and hypochlorite bleach work by chlorination and leave chlorinated by-products behind. The names are similar and the chemistry is not.
- What is chlorine dioxide used for?
- Municipal drinking water treatment, food and beverage processing, pulp bleaching, and on a smaller scale the disinfecting of water storage — RV and marine tanks, wells, stored drinking water.
- Why does it come as two bottles instead of one?
- Because chlorine dioxide in solution does not keep. It is generated on the spot from a stabilised precursor and an activator, which is what the two bottles are. A single bottle of ready-made ClO₂ at working strength would be spent before it reached you.
- Is chlorine dioxide safe to drink as a medicine?
- No, and we sell it for no such purpose. Products marketed as MMS or 'Miracle Mineral Solution' are concentrated chlorine dioxide solutions sold as a cure for illness; the FDA has warned against drinking them and they have caused serious harm. What is on this site is for treating and preserving water, used at the dilutions the label gives.
- Does chlorine dioxide leave a taste?
- Far less than chlorine does. The 'swimming pool' taste comes from chlorophenols, which form when chlorine meets phenols in the water. Chlorine dioxide does not produce them.