The dynamics of the unimolecular decomposition of the methanethiol radical cation (CH3SH+) was investigated by direct classical trajectories in which the interatomic forces were evaluated by semi-empirical AM1 calculations. At 170 kcal/mol, the methanethiol cation decomposition is found to be non-statistical. The abundances of product ions calculated for different initial conditions are in reasonable agreement with those determined experimentally. Our results corroborate that the coupling between the low- and high-frequency vibrational modes is weak in the methanethiol cation.