The selective formation of methane from the decomposition of methanethiol increases as the coverage of Ni thin films is increased from 0 to 2 monolayers (RIL) on W(001). The methane selectivity starts at 30% on the clean W(001) surface, rises to 70% when the Ni coverage is 1 ML, dips to 60% at 1.5 ML, and then continues to rise to >85% when the Ni coverage is 2 ML. This is the largest selectivity for methane formation yet reported for this reaction in ultrahigh vacuum. Accompanying the increase in selectivity is a sequential decrease in the methane desorption temperature. The methane desorbs at 350 K from methanethiol on clean W!001), at 290 K from a 1 ML Ni film, and at 210 K from a 2 ML Ni film. On thick (>5 hin), unannealed films, the desorption temperature increases to 270 K and the selectivity is reduced to 70%. S 2p soft X-ray photoemission indicates that the C-S bond is cleaved between 300 and 350 K on clean W(001) and between 250 and 300 K on a 1 ML Ni film. On the 2 ML Ni film, there is considerable C-S bond cleavage (>50%) at 100 K. The only molecular intermediates are methyl thiolate (CH3S) and hydrocarbon fragments. The methyl thiolate forms at 100 K at all Ni coverages. An amount of 65% of the thiolate occupies low coordination sites on clean W(OO I), while most of the thiolates occupy high coordination sites on the Ni-covered sur-faces.