Sulfiding of bulk and alumina-supported vanadium oxides has been studied using temperature-programmed sulfiding and reduction techniques. Bulk compounds (V2O5, V2O3) and V/Al2O3 catalysts are sulfided via a similar mechanism. For bulk V2O5, two major sulfiding steps have been identified. At temperatures up to 673 K, V2O5 is reduced to V2O3 by O-S exchange and subsequent rupture of V-S bonds where H2S acts as reducing agent. Sulfiding to V2S3 takes place above 673 K. The catalysts are sulfided more easily than the bulk oxides due to the higher dispersion of the vanadium species. In catalysts sulfided at 673 K which are still partially oxidic, four types of sulfur have been observed, viz, adsorbed H2S, stoichiometric sulfur, S-H groups, and nonstoichiometric (excess) sulfur (S-x). There are indications that (isothermal) room temperature H2S adsorption can be used to determine the dispersion of the supported microcrystallites at higher vanadium loadings. From the present results it is inferred that alumina-supported vanadium-based catalysts, when sulfided at temperatures commonly applied in hydrotreating operations, essentially consist of an oxide, the outer surface of which is sulfided.