High resolution XPS, HREELS, and TPD data indicate two distinct binding modes for methyl thiolate with clearly distinct reactivites on the Ni(111) surface. At low temperatures methyl thiolate is adsorbed primarily at low coordination, probably bridging, sites with the C-S bond tilted toward the surface and undergoes hydrogenolysis near 275 K. Between 150 and 250 K, a fraction of the bridge bonded thiolates are converted to thiolates in hollow sites with their C-S bonds oriented nearly perpendicular to the surface. The C-S bond in the hollow bonded thiolate is more stable and does not undergo hydrogenolysis until around 300 K. Overall, about 80% of the chemisorbed thiolates undergo hydrogenolysis (an overall disproportionation reaction) to form methane in the 250-350 K temperature range. The remaining 20% of the thiolates are completely dehydrogenated to form adsorbed atomic carbon and sulfur. Coadsorbed hydrogen favors formation of the thiolate in the bridge site. In the presence of coadsorbed deuterium, a substantial fraction of the methane formed is multiply deuterated (-d(2) and higher); however, no kinetic isotope effect is observed for the methane formation reaction. These results suggest C-S bond breaking as the rate determining step for methane formation.