The isomerization reaction of CH₃S→CH₂SH (R1), H₂ elimination reaction of CH₃S→HCS＋H₂ (R2) and the isomerization reaction of HCS→CSH (R3) have been studied by using the density functional theory. Geometries, harmonic vibrational frequencies of all stationary points and the minimum energy paths for the title reactions were calculated at the MPW1PW91 level in conjunction with the 6-311G(d,p) basis set. In order to obtain more reliable energies, higher level energy calculations for the stationary points and selected points were carried out at the QCISD(t)/6-311＋＋G(d,p)//MPW1PW91/6-311G(d,p)＋ZPE levels. The rate constants of the reactions were evaluated by means of the classical transition state theory, the canonical variational transition state theory, and canonical variational transition state theory incorporating small-curvature tunneling correction in the temperature range of 200～2000 K. The results show that the energy barriers of the reactions R1, R2 and R3 △E^≠ are 160.69, 266.61 and 241.63 kJ/mol, respectively, and the reaction R1 is the dominant reaction channel. CH₃S radical is more stable than CH₂SH in low temperature region, while the latter is more stable in high temperature region. The tunneling effect is significant and the variational effect is small for the calculated rate constants.

Key words: methylthio radical, isomerization reaction, H₂ elimination, rate constant