The reactivity of 1,2-ethanedithiol on the clean Mo (110) and p(4X4)-C/Mo (110) surfaces has been investigated as a function of sulfur coverage using temperature programmed desorption (TPD), Auger electron spectroscopy, and low energy electron diffraction. TPD experiments performed on both surfaces produced similar reaction products, although changes were observed in selectivity. On the clean Mo (110) surface, the major products observed during TPD experiments were acetylene, ethylene, vinyl thiol, and ethanethiol. However, the reaction of ethanedithiol on the p(4X4)-C/Mo (110) surface produced acetylene, ethylene, and ethanedithiol. Product molecules are thought to arise from two distinct types of surface intermediates : (1) a monodentate thiolate species bound to the surface through only one of the ethanedithiol sulfur atoms, and (2) a bidentate organosulfur metallocycle bound to the surface through both of the ethanedithiol sulfur atoms. We propose that vinyl thiol and ethanethiol are produced via C-S bond scission and subsequent hydride elimination of the monodentate thiolate intermediate, and that the bidentate surface metallocycles undergo C-S bond scission to yield acetylene and ethylene. On the carbon-modified surface, complete desulfurization of ethanedithiol occurs upon decomposition, yielding only hydrocarbon products. With increasing sulfur coverage, a decrease in reactivity and a shift in desorption features to lower temperatures is observed for ethanedithiol on the clean and carbon-modified surfaces.