NiMo catalysts were prepared on gamma-Al2O3 modified with SiO2 (0 and 10 wt% SiO2). Catalysts were characterized by HRTEM in sulfided state and evaluated in the hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT), hydrogenation (HYD) of naphthalene, hydrodenitrogenation (HDN) of carbazole and in the simultaneous reaction of 4,6-DMDBT-naphthalene-carbazole. The kinetic parameters of Langmuir-Hinshelwood type equations for all the reaction systems were estimated. Although naphthalene and carbazole present a competitive inhibition effect for the HDS of 4,6-DMDBT, the inhibiting effect of naphthalene was mainly due to its high concentration, whereas the inhibition caused by carbazole was due to its strong adsorption over the catalytic active sites. The addition of SiO2 to the catalysts enhanced the hydrogenation activity and therefore the HDS of 4,6-DMDBT, HYD of naphthalene and HDN of carbazole leading to more active catalysts when a mixture of the three compounds was hydrotreated. (C) 2007 Elsevier B.V. All rights reserved.