To develop an active and selective catalyst for the selective catalytic reduction of SO2 to elemental sulfur with hydrogen, various transition metal sulfides supported on gamma-Al2O3 were studied. All the catalysts yielded high selectivities of sulfur regardless of the SO2 conversion achieved. It was believed that the SO2 hydrogenation to H2S that occurs over the metal sulfide phase was much slower than the Claus reaction (2H(2)S + SO2 reversible arrow S + 2H(2)O) over alumina support. Simply increasing the loading of the metal sulfide did not enhance the (overall) conversion of SO2. Among the catalysts tested, Fe group metals (Fe, Co, Ni) showed the highest catalytic activity (conversion of SO2). These metal sulfides experienced a phase transformation from a sulfur-deficient form to a catalytically active disulfide form (FeS2, CoS2, and NiS2) during the reaction, The volcano curves obtained by plotting the SO2 hydrogenation activity with physicochemical properties (heat of formation and bond dissociation energy) suggested that the active sites for the SO2 hydrogenation were sulfur vacancies.