Sodium thiosulfate is an inorganic compound with the formula Na2S2O3.xH2O that dissolves well in water.

Sodium Thiosulphate solution is used to de-chlorinate tap water including lowering chlorine levels for use in aquariums, swimming pools, and spas and within water treatment plants to treat settled backwash water prior to release into rivers.

In pH testing of bleach substances, sodium thiosulfate neutralizes the colour-removing effects of bleach and allows one to test the pH of bleach solutions with liquid indicators. The relevant reaction is akin to the iodine reaction: thiosulfate reduces the hypochlorite (active ingredient in bleach) and in so doing becomes oxidized to sulphate.

Solutions of sodium thiosulfate are commonly used as a precaution in chemistry laboratories when working with bromine and for the safe disposal of bromine, iodine, or other strong oxidizers.