The radical HSO is an oxidation product of pollutants such as H2S and CH3SH in Earth's atmosphere. For the first time, the interaction of HSO and its tautomer HOS with single water molecules, to yield the hydrates HSO center dot nH(2)O and HOS-nH(2)O was studied for n = 1-3, applying the high-level G3X(MP2) theory. A large number of structures corresponding to local minima on the potential energy surfaces has been identified. While gaseous HSO is more stable than HOS, the enthalpy difference between HSO center dot nH(2)O and HOS center dot nH(2)O decreases with increasing degree of hydration and becomes practically zero for n = 3. Thus, in aqueous solution as well as in fog and rain droplets, HOS is expected to compete with HSO. The barrier for the tautomerization of HSO to HOS is dramatically lowered by the presence of water molecules since a cyclic transition state allows a concerted proton shift within the system of neighboring hydrogen bonds. The corresponding activation enthalpy of only 73.5 kJ mol(-1) predicted for the transformation of HSO center dot 2H(2)O into HOS center dot 2H(2)O may be compared to the 202 kJ mol(-1) reported for the transformation of the H2O center dot 2H(2)O into HSO/HOS molecules. The impact of water of hydration on the fundamental vibrational modes of HSO and HOS has also been studies. Furthermore, HOS is predicted to dimerize at low temperatures to give two van der Waals molecules with singlet (symmetry C-2) or triplet configuration (symmetry C-2h), the latter being more stable than the singlet isomer. The disproportionation of 2HSO to H,S and SO, is predicted to be exothermic by -263.5 kJ mol(-1). The reaction of HSO with ozone to HSO2 and O-2 is also strongly exothermic by -274.0 kJ mol(-1) and seems to proceed without any barrier. HOS forms a 1:1 van der Waals complex with O-3; the redox reaction of its two components is calculated as exothermic by -410.9 kJ mol(-1) and results in a rather stable adduct between HOSO and O-2 with the structure of a peroxo isomer of HOSO3. This unprecedented hydrogen peroxosulfite radical might open a novel route to atmospheric sulfate without the intermediate formation of SO2 and SO3.