The tetrathionate anion, S4O62−, is a sulfur oxoanion derived from the compound tetrathionic acid, H2S4O6. Two of the sulfur atoms present in the ion are in oxidation state 0 and two are in oxidation state +5. Alternatively, the compound can be viewed as the adduct resulting from the binding of the Lewis base S22− to SO3. Tetrathionate is one of the polythionates, a family of anions with the formula [Sn(SO3)2]2−.

Tetrathionate is a product of the oxidation of thiosulfate, S2O32−, by iodine, I2:

2S2O32− + I2 → S4O62− + 2I


Tetrathionate's structure can be visualized as following three edges of a cube, as in the diagram below. The structure shown is the configuration of S4O62− in BaS4O6·2H2O and Na2S4O6·2H2O. Dihedral S-S-S angles approaching 90° are common in polysulfides.


Compounds containing the tetrathionate anion include sodium tetrathionate, Na2S4O6, potassium tetrathionate, K2S4O6, and barium dithionate dihydrate, BaS4O6·2H2O.


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