The adsorption and thermal decomposition of methanethiol (CH3SH) on Ag(110) and on Cu(110) surfaces have been investigated by low energy electron diffraction, Auger electron spectroscopy, and temperature programmed desorption methods. On the clean Ag(110) surface, methanethiol nondissociatively adsorbs at 25 K, forming a (2 x 1) overlayer structure and then molecularly desorbs at similar to140 K. On the partially sulfur-covered Ag(110) surface, methanethiol readily decomposes below 240 K, producing CH4 (g), H2S (g), and sulfur atoms on the surface as well as small amounts of carbon. The sulfur overlayer formed by the decomposition of methanethiol exists as a p(3 x 2) ordered structure at the saturation coverage of sulfur. On this fully sulfur-covered Ag(110) surface, the thermal decomposition of methanethiol is completely suppressed. An autocatalytic mechanism involving silver sites activated by neighboring adsorbed sulfur is proposed to explain the sulfur-catalyzed thermal decomposition reaction of methanethiol on Ag(110). In contrast, on the clean Cu(110) surface, methanethiol decomposes below 320 K, producing CH4 (g) and C2H6 (g), and leaving sulfur overlayers, either a c(2 x 2) structure at low coverage or a c(8 x 2) structure at high coverage. A complete layer of chemisorbed sulfur suppresses further methanethiol decomposition on both the silver and the copper surfaces.