The chemisorption of methanethiol an W(001) resulting in the formation of methyl thiolate decreases linearly as a function of preadsorbed atomic S. The ratio of selective decomposition of methyl thiolate, which results in methane desorption, to total decomposition, resulting in desorbed H-2 and adsorbed atomic S and C. remains constant. The S 2p photoemission from methanethiol on W(001) consists of peaks from two methyl thiolate species. The presence of atomic S adsorbed in fourfold hollow sites preferentially inhibits the thiolate species with the higher S 2p binding energy. This suggests that this thiolate species is also adsorbed in a fourfold hollow, and its adsorption is blocked by atomic S. Adsorption of thiolate on the S-precovered surface causes the S 2p peak position of the atomic S to shift to lower binding energy. This is the result of crowding caused by the chemisorption of the thiolate in adjacent Bites but is not due either to direct bonding of the thiolate to the preadsorbed S or to a major reconstruction of the surface.