The adsorption of methanethiol on the clean Cu(110) and the sulfur-passivated Cu(110) surface has been studied with the time-of-flight electron-stimulated desorption ion angular distribution (TOF-ESDIAD) method, temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). Methanethiol on the clean Cu(I 10) surface thermally decomposes below 320 K, producing CH4(g) and C2H6(9), leaving the sulfur atom on the surface. A c(2 x 2) LEED pattern was observed at a low coverage of sulfur, and a c(8 x 2) structure developed from the c(2 x 2) phase at the saturation coverage of sulfur. Chemisorbed sulfur acts as a poison for further methanethiol decomposition on the surface. On the sulfur-passivated Cu(110) surface, methanethiol adsorbs dissociatively and produces a methanethiolate layer on the surface. The molecular orientation of CD3S on the sulfur-passivated Cu(110) surface was determined by assessing the C-D bond direction with respect to the surface normal and the crystal azimuths using the TOF-ESDIAD method. The C-S bond direction in the methanethiolate on the sulfur-passivated Cu(110) surface was oriented parallel to the (001) azimuthal plane. The C-S bond axis of methanethiolate is tilted 45degrees away from the surface normal.