The changes in morphology, structure of active sites, dispersion, sulfidation extent, rate constant, turnover frequency, and selectivity in the hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzotiophene, were analyzed when citric acid was added to Mo/Al2O3 and CoMo/Al2O3 catalysts. For unpromoted catalysts, citric acid induces a decrease in the number of adsorbing sites, better sulfidation, and increased CO adsorption in S-edges. These changes have little influence in the turnover frequency or in the selectivity of hydrodesulfurization of dibenzothiophene. In Co-promoted catalysts, citric acid induces better sulfidation and promotion, and increased surface reducibility, number of Co-Mo-S sites, and Co in square planar local structure. From all these changes, the increased number of promoted sites is the major contributor to the improvement in rate constant for hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzotiophene. The higher sulfidation level needed to produce type II and brim hydrogenating sites did not affect significantly the catalyst activity. (C) 2014 Elsevier Inc. All rights reserved.