Alumina-supported molybdenum sulfide catalyst was prepared by the decomposition of ammonium tetrathiomolybdate (ATTM) and tested in the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The HDS activity of unpromoted and Co-promoted molybdenum sulfide catalysts was found to depend strongly on the activation procedure of the catalyst. In all cases, the activity of the Go-promoted catalyst was superior to that of the ATTM counterpart. Surface characterization of the catalyst subjected to different pretreatments was carried out by X-ray photoelectron spectroscopy and FTIR spectroscopy of NO probe. The pretreatments were effective in obtaining a highly dispersed heterometallic sulfide phase and a good synergy between Co and molybdenum sulfide.