Ab initio MO calculations at the MP2/6-311G** level of theory show that the thiosulfoxide H2SS is 143 kJ mol(-1) less stable than the disulfane HSSH, separated by an activation barrier of 210 kJ mol(-1). Using much higher levels of theory, these results are only slightly changed, proving the suitability of the economical MP2/6-311G** scheme for the present purpose. At the same level Me(2)SS is 84 kJ mol(-1) less stable than MeSSMe, with the transition state lying 340 kJ mol(-1) above MeSSMe. Thermal isomerization of HSSH or MeSSMe can therefore be excluded, and H2SS and Me(2)SS should be kinetically stable toward unimolecular isomerization at low temperatures, Vibrational wavenumbers for H2SS and Me(2)SS are given. The allyl methyl thiosulfoxide MeAllSS is 83 kJ mol(-1) less stable than the disulfane MeSSAll, but isomerization of the latter requires only 110 kJ mol(-1). The corresponding RSSR and R(2)SS structures for R = Pr and All have been calculated at the HF/6-31G* level. The bimolecular decomposition of dimethyl thiosulfoxide to dimethyl sulfide and S-2 is exothermic but spin-forbidden in the case of triplet S-2, and endothermic but spin-allowed when singlet S-2 is formed. The reactions of Me(2)SS with sulfur molecules S-x (x > 2) to give Me(2)S and S-x+1 are exothermic.