Kinetic, isotopic, and chemical analysis methods are used to examine the identity and kinetic relevance of elementary steps and the effects of Pt cluster size on thiophene hydrodesulfurization (HDS) turnover rates. Quasi-equilibrated H2 and H2S heterolytic dissociation steps lead to sulfur chemical potentials given by the prevalent H2S/H2 ratio and to cluster surfaces with a metallic bulk, but near-saturation sulfur coverages, during steady-state catalysis. Sulfur-vacancies on such surfaces are required for η1(S) or η4 thiophene adsorption modes and for H2 and H2S dissociation steps. H-assisted CS bond cleavage of η1(S) thiophene and H-addition to η4 thiophene limits rates of direct desulfurization and hydrogenation sulfur removal pathways, respectively. These steps, their kinetic relevance, and the prevalent sulfur-saturated surfaces resemble those on Ru clusters; they are also consistent with the observed kinetic effects of reactants and products on rates, with the rapid isotopic exchange in H2/D2/H2S mixtures during HDS catalysis, and with measured H2/D2 kinetic isotope effects. Small Pt clusters exhibit lower turnover rates, stronger inhibition by H2S, and a greater preference for desulfurization pathways than those of large clusters. These effects reflect the prevalence of coordinatively unsaturated corner and edge sites on small clusters, which bind sulfur atoms more strongly and lead to lower densities of vacancies and to a preference for η1(S)-bound thiophene species. Sulfur binding energies and their concomitant effects on the number of available vacancies also account for the higher turnover rates measured on Pt clusters compared with Ru clusters of similar size. These data and their mechanistic interpretation suggest that the concepts and steps proposed here apply generally to hydrogenation and direct desulfurization of organosulfur compounds. Taken together with similar observed effects of oxygen binding strength, metal identity, and cluster size for oxidation reactions of NO, hydrocarbons, and oxygenates, which also require vacancies in their respective kinetically relevant steps, these data also indicate that low reactivity of small clusters may reflect in most instances their coordinative unsaturation and the concomitant kinetic and thermodynamic preference for low vacancy concentrations on nearly saturated surfaces.