A series of CoMo HDS catalysts supported on alumina with different amounts of aluminum-modified MCM-41 was prepared, characterized by different techniques (S-BET, NH3-TPD, TPR, UV-VIS-DRS, HREM and NO adsorption analyzed by FTIR) and tested in the dibenzothiophene hydrodesulfurization reaction. The results show that the incorporation of MCM-41 to the catalyst formulation leads to higher catalytic activity. The TPR results indicate a diminished interaction of the Co and Mo phases with the support, compared to those existing in the alumina-supported catalyst. This decrease in the interaction causes the formation of greater amounts of polymeric Mo species and a decrease in the population of tetrahedral Co as CoAl2O4, as evidenced by the DRS results. The changes in the intensity and position of the NO adsorption LR bands indicate that an interaction between Co and Mo exists and that not all the Co present in the catalysts is completely sulfided, and therefore, may not contribute to the formation of the catalytically active Co-Mo-S mixed phase.