Monometallic, Mo, and bimetallic, Ni-Mo, catalysts supported on zirconia have been prepared and characterized by X-ray photoelectron spectroscopy (XPS). These catalysts were also tested in the hydrodesulfurization of a vacuum distillate in a batch autoclave at 330 degrees C and 60 atm. The investigation by XPS of the oxidic catalysts with different Mo loadings showed that most likely a growth of 3D MoO3 crystallites takes place. The investigation of the sulfided catalysts showed that they contained molybdenum in a formal 4(+) oxidation state. The quantitative analysis suggested that the sulfidation was within the experimental uncertainty complete and no structural changes occurred in the Mo phase. The comparison between monometallic and bimetallic catalysts indicated a strong enhancement of the molybdate species dispersion in the latter case, which leads to the conclusion that the main role of the Ni promoter is the improved spreading of the active phase. The strongest dispersion enhancement occurs for low MoO3 loadings (7% MoO3) and this is consistent with measurements of the catalytic activity whereby the 7% MoO3 catalyst exhibited the largest enhancement of the catalytic activity for the hydrodesulfurization reaction.