Kinetic and isotopic methods were used to probe elementary steps and site requirements for thiophene hydrogenation and desulfurization on Ru metal clusters. Turnover rates for these reactions were unaffected by whether samples were treated in H-2 or H2S to form metal and sulfide clusters, respectively, before reaction. These data, taken together with the rate and extent of sulfur removal from used samples during contact with H-2, indicate that active structures consist of Ru metal clusters saturated with chemisorbed sulfur at temperatures, pressures, and H2S levels relevant to hydrodesulfurization catalysis. Turnover rates and isotopic data over a wide range of H-2, H2S, and thiophene pressures are consistent with elementary steps that include quasi-equilibrated H-2 and H2S heterolytic dissociation and thiophene binding with eta(1)(S) or eta(4) coordination onto sulfur vacancies. We conclude that hydrogenation proceeds via addition of protons (H delta+, as -S-H delta+ from H-2 or H2S dissociation) to eta(4) thiophene species, while desulfurization involves C-S activation in eta(1)(S) species aided by H delta- species formed via H-2 dissociation. Reactant concentrations influence hydrogenation and desulfurization turnover rates to the same extent, suggesting that the involvement of similar active structures, consisting of vacancies on sulfur-covered Ru clusters. Smaller turnover rates and stronger H2S inhibition on smaller Ru clusters for hydrogenation and desulfurization routes reflect the stronger sulfur binding and the smaller vacancy concentrations on small clusters, which contain exposed atoms with lower average coordination. A preference for eta(1)(S) over eta(4) thiophene species at the higher sulfur coverages that prevail on smaller Ru clusters causes desulfurization and hydrogenation rate ratios to increase with decreasing cluster size. We conclude that hydrogenation and desulfurization routes require similar active sites and that weaker M-S bonds lead to higher concentrations of kinetically-relevant sulfur vacancies. These elementary steps and site requirements are likely to also prevail on metals and sulfides with M-S bond strengths similar or higher than Ru-S, for which vacancy sites are also present as minority species. (C) 2010 Elsevier Inc. All rights reserved.