The unimolecular decay of the triplet thiomethoxy cation CH3S+, ion 1, has been investigated by density functional theory, ab initio, and Phase-space/Rice Ramsperger Kassel Marcus (PST/RRKM) calculations. We have first located on the singlet and triplet B3LYP/6-311+G(d,p) [C,H-3,S](+) potential energy surfaces the energy minima and transition structures involved in the lowest energy decompositions of 1, including the loss of H, H-2, and S. We have subsequently located the minimum energy points lying on the B3LYP/6-311+G(d,p) hyperline of intersection between the singlet and triplet surfaces, using a recently described steepest descent-based method [Theor. Chem. Acc. 99, 95 (1998)]. The total energies of all these species were refined by CCSD(T)/cc-pVTZ single-point calculations. The obtained potential energy surface has been used to outline the full kinetic scheme for the unimolecular decay of ion 1. The rate constants of the various elementary steps have been calculated by the PST and the RRKM theory. We used a nonadiabatic version of the latter to evaluate the rate constants of the elementary steps which involve a change in the total spin multiplicity. We found that the two kinetically favored decomposition channels are the loss of atomic hydrogen, with formation of (CH2S+.)-C-2, and molecular hydrogen, with formation of (HCS+)-H-1. The former process is predicted to prevail for ions 1 in the lowest rotational states and with an internal energy content of at least 60 kcal mol(-1). The loss of H-2 was found to be by far the prevailing process in the time scale of ca. 10(-5) to ca. 10(-6) s from the formation of 1. This is fully consistent with the experimentally observed exclusive loss of H-2 by the CH3S+ ions which decompose in the "metastable" time window of the mass spectrometer. The loss of H-2 from ion 1 with formation of (HCS+)-H-1 may occur by two distinct "spin-forbidden" paths, i.e., a simple concerted 1,1 H-2 elimination or a 1,2 H shift followed by a 1,2 H-2 elimination from the singlet mercaptomethyl ion 2. In the metastable time window, these two mechanisms may occur alternatively, depending on the degree of rotational excitation of 1.