We present studies of the photoinduced surface chemistry of methanethiol adsorbed on GaAs(110). UV irradiation of these molecules, which are intact at similar to90 K, induces scission of the S-H. bond, leaving the CH3S fragment trapped at the surface. The measured cross section at 193 nm for this photoinduced process is similar to3.6 x 10(-10) cm(2). Postirradiation heating of the surface-bound products of the methanethiol photoreaction results in the evolution of dimethyl sulfide with the same desorption kinetics as observed for the strictly thermal production of dimethyl sulfide from dissociatively adsorbed methyl disulfide. Thus, UV irradiation of CH3SH/GaAs(110) followed by thermal activation of the surface-trapped intermediates is shown to provide a new route to partial desulfurization of CH3SH.