The reactions of methanethiol on cobalt overlayers grown on Mo(110) were studied using a temperature-programmed reaction and X-ray photoelectron and high-resolution electron energy loss spectroscopies. Methyl thiolate was identified as the reaction intermediate on the basis of X-ray photoelectron and high-resolution electron energy loss data. Methane, methyl radical, and H-2 were the only gaseous products. The peak temperature for methane production from methyl thiolate hydrogenolysis was relatively insensitive to the Co coverage and geometric structure of the Co layer. However, less methyl radical formation was observed as the Co coverage increased from 1 to 2.5 monolayers. The selectivity for hydrocarbon formation was essentially the same, similar to 48 +/- 5%, for all Co coverages. The total amount of methyl thiolate deposited in the initial S-H bond breaking was independent of Co coverage. At 400 K, sulfur-induced structural rearrangement of the Co overlayer was insignificant and therefore did not affect the reaction of methanethiol. The mixed Co-S overlayer had a substantially lower activity for the methanethiol reaction than any of the clean surfaces; the total amount of reaction on both the sulfur and Co-S overlayers was 30% that of the clean Mo(110) and pure Co overlayers.