The genesis of sulfur vacancies on sulfided (Co)Mo/Al2O3 catalysts was studied with EXAFS at the reaction temperature (673 K) in a H-2 and H-2/thiophene gas atmosphere. For Mo no significant changes in the sulfur coordination were observed for either the Go-promoted or the unpromoted sample. The experiments indicated that in Go-promoted Mo/Al2O3 vacancies are primarily formed on the Co atom. The sulfur coordination of the cobalt changed from 6.0 to 5.2 upon reduction with H-2, followed by a slight increase to 5.4 by treatment with H-2/thiophene. The reaction of selenophene (a structural analogue of thiophene) in hydrogen with sulfided (Co)Mo/Al2O3 catalysts was also investigated with EXAFS. It was possible to monitor the incorporation of Se in the metal sulfide phase. Characterization of the promoted catalyst after HD-Se at 473;K showed that Se was exclusively coordinated to the Co atoms. In contrast, data obtained after HD-Se at 673 K revealed that the Se atoms were located in the position of the bridging sulfur atoms between Co and Mo. Accordingly, a structural model is proposed that involves two types of active sites for hydrodesulfurization on the sulfided Go-promoted Mo catalyst. The first type consists of a sulfur vacancy that is only associated with the promoter atoms, which is created at low temperatures. At higher temperatures, a second type of site is produced by removal of sulfur atoms that are bonded to both Co and Mo atoms.