The reaction selectivities and bonding configurations in mercaptoethanol (HSCH2CH2OH) and ethanedithiol (HSCH2CH2SH) on Ni(110) were determined and found to be correlated to the relative strengths of the C-S and C-O bonds. Mechanistic details of the mercaptoethanol reaction have been elucidated. Mercaptoethanol reacts with Ni(110) to form ethanol, acetaldehyde, methane, CO, and hydrogen. Mercaptoethanol reactivity is dominated by thiolate formation as suggested by the S 2p core level binding energy. Near 200-250 K, the C-S bond cleaves with the evolution of ethanol and formation of surface ethoxide which is easily discernible from vibrational spectroscopy. The ethoxide dehydrogenates to form acetaldehyde, which either desorbs or reacts to form methane and CO. In dramatic contrast, the structurally similar ethanedithiol apparently adsorbs in a bidentate fashion, resulting in a dithiolate which selectively splits out ethylene and produces surface sulfur. The difference in the reactivity of ethanedithiol and mercaptoethanol is understood in terms of the inactivity of the Ni(110) surface toward C-O bond scission, which is primarily a reflection of the strength of C-O bonds compared with C-S bonds.