High-level ab initio MO calculations have been performed to determine the structures, energies, and vibrational spectra of eight isomers (including rotamers) of H2S2O and five isomers/rotamers of Me(2)S(2)O. At the MP2/ 6-311G**//MP2/6-311G** level of theory, including the zero-point energies (ZPE), the order of decreasing stability of H2S2O molecules is trans-HOSSH > cis-HOSSH > HOS(S)H > trans-HSS(O)H > cis-HSS(O)H > trans-HSOSH > cis-HSOSH > HS(O)(S)H. The remarkable stability of the thiono derivative HOS(S)H over the sulfoxide HSS(O)H is explained by the stabilization of the former by an intramolecular OH...S hydrogen bond. The dimethyl derivatives of the more stable H2S2O isomers have been calculated at the MP2/6-311G**//HF/6-311G**+ZPE level. The order of decreasing stability is trans-MeOSSMe > cis-MeOSSMe > cis-MeSS(O)Me > trans-MeSS(O)Me > MeOS(S)Me. The geometrical parameters of these species agree well with those of related compounds, e.g., MeOSSOMe and RSS(O)R with = p-tolyl.