The influence of Co on the acidic properties (Lewis and Bronsted) of molybdenum sulfide catalyst were determined by infrared studies of adsorbed pyridine or 2,6-dimethylpyridine, These studies show that the promoter addition increases the strength of the Lewis sites and the number of Bronsted sites, These results are correlated to those obtained for dibenzothiophene conversion. For this reaction, it was observed that the influence of the promoter is very different for the two different routes involved in the reaction scheme. Consequently, the role of the promoter is not only to increase the number of sulfur vacancies but also to modify the acid-base properties of the catalyst.