In the present work, we studied the effect of the addition of niobium to CoMo/MCM-41 and NiM0/MCM-41 catalysts on their characteristics and performance in the hydrodesulfurization of dibenzothiophene. The MCM-41 support was synthesized at room temperature according to a well-known procedure. The metal species (Nb, Mo, Ni or Co) were deposited by successive impregnation using aqueous solutions of the corresponding metal salts. MCM-41 and Nb-containing MCM-41 supports were characterized by X-ray diffraction, N-2 physisorption, FT-Raman spectroscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy and scanning electron microscopy. The CoMo and NiMo catalysts in oxide state, in addition to the above techniques, were characterized by temperature-programmed reduction and the sulfided catalysts were characterized by high resolution transmission electron microscopy. The results show that the incorporation of small amounts of Nb (3-5 wt.%) increased the catalytic activity of both NiMo and CoMo catalysts in dibenzothiophene hydrodesulfurization and affected their selectivity. The effect of Nb on the selectivities of the NiMo and CoMo catalysts was different: for the NiMo catalysts, Nb increased selectivity towards the HYD route, whereas for the CoMo catalysts, it increased selectivity towards the DDS route. The analysis of the effect of Nb on the catalysts' selectivity was performed based on the kinetic constants calculated for the different steps of the DBT HDS network. (C) 2017 Elsevier B.V. All rights reserved.