Density functional theory was used to investigate the reaction pathways for HSCH3 adsorption on Au(111) at low coverage. A molecular adsorbed state was found with the S atom bond on Top sites (E similar to -0.38 eV) and molecular adsorption is nonactivated. The H-SCH3 dissociation process is energetically less favorable and becomes slightly exothermic only when surface relaxation is considered (Delta E similar to -0.2 eV). All the reaction pathways present a sizable activation energy barrier, with the lowest being similar to 0.52 eV (0.41 eV taking into account slab relaxation). In the corresponding saddle point of the potential energy surface, the S atom of the methylthiolate molecule is placed on Top sites and the H near a Bridge site. The high barrier obtained explains the complete absence of reactive methanethiol dissociation found in recent experiments.