The interaction of thiosulfuric acid and its monoanion with up to three water molecules has been studied by density functional and high-level ab initio calculations, More than 40 molecules and anions both as OH and SH tautomers were investigated. The structures in the gas phase as well as in a polarizable continuum were optimized at the B3LYP/6-31G(2dfp) level of theory, whereas G3X(MP2) single-point calculations were applied to obtain enthalpies and Gibbs energies of the gaseous species. In the gas phase, all monoanions of composition [H,S-2,O-3]-center dot nH(2)O (n = 0-3) are predicted to be most stable as SH tautomers [HSSO3](-). The enthalpies of hydration are -51 +/- 5 kJ mol(-1) per water molecule. In a polarizable phase simulating the dielectric properties of water the OH forms [SSO2(OH)](-) of the hydrated and unhydrated rnonoanions are always most stable, Thiosulfuric acid and its mono- and dihydrates are most stable in the SH/OH form, both in the gas phase and in the polarizable continuum. However, the trihydrate H2S2O3 center dot 3H(2)O prefers the OH/OH form in the polarizable phase, whereas in the gas phase the SH/OH tautomer represents the lowest minimum structure. The hydration enthalpy of the acid is slightly smaller than predicted for the monoanions. We predict that both thiosulfuric acid and its monoanion exist as equilibrium mixtures of the corresponding tautomers in aqueous solution. The acid decomposition of thiosulfate involves sulfur transfer reactions, but the formerly accepted reaction between two rnonoanions producing sulfite and [HSSSO3](-) ions is endothermic and endergonic both for the naked anions and their trihydrates. Therefore, we propose that the sulfur transfer takes place between [HSSO3](-) and H2S2O3 producing [HSSSO3](-), SO2, and H2O. This reaction is exothermic and exergonic both in the gas phase and in the polarizable phase. From [HSSSO3](-) the longer chain sulfane monosulfonate ions [HSnSO3](-) are formed by a series of sulfur transfer reactions, and these ions eventually split off homocyclie sulfur molecules S, The initial decomposition reaction is hindered by SO2 which reacts with thiosulfate ions to the novel adduct [O2SSSO3](2-) which seems to be also a key intermediate in the synthesis of trithionate from thiosulfate and SO2. The implications of these results for the enzymatic thiosulfate metabolism by sulfur bacteria are discussed. The adduct H2S center dot SO3, an isomer of thiosulfuric acid, has been studied in addition. Its conversion into H2S2O3 is predicted to be exothermic, by -39.5 kJ mol(-1).