The tetrathionate anion, , is a sulfur oxyanion derived from the compound tetrathionic acid, HSO. 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 disulfide| to SO.  Tetrathionate is one of the polythionates, a family of anions with the formula [S(SO)]. Its IUPAC name is 2-(dithioperoxy)disulfate, and the name of its corresponding acid is 2-(dithioperoxy)disulfuric acid. The Chemical Abstracts Service identifies tetrathionate by the CAS Number 15536-54-6.