Mo/TiO2 catalysts were modified with Nb by two different methods, sol-gel and surface deposition, in order to study the effect of Nb incorporation on the thiophene HDS activity. The results show that the formation of Nb-Ti mixed oxides leads to catalysts with poor HDS activity while the deposition of Nb oxide species on the surface of TiO2 leads to catalysts with activities larger than those of Mo/Al2O3 and Mo/TiO2. This increase in activity was attributed to the formation of a larger population of Mo sulfur anionic vacancies when Nb was surface deposited on the TiO2.