A series of mechanical mixtures Mo/A1(2)O(3)-TiO2/Al2O3 has been prepared to study the role of titania in Mo-based HDS catalysts. The catalysts were tested in the thiophene hydrodesulfurization reaction at atmospheric pressure and were characterized by UV-vis diffuse reflectance spectroscopy (DRS), TPR-S (after reaction), and high-resolution electron microscopy (HREM). The results show that the titanium species formed during sulfidation and HDS reaction act as promoter of the Mo phase giving rise to a synergy effect, This synergy effect seems to be related to the electronic properties of the partially reduced and/or sulfided TiO2 surface and, in particular, to the presence of Ti3+ ions. Finally, estimation of the number of sulfur vacancies created on the used catalysts during TPR-S indicate that only a small proportion of all the edge/corner Mo atoms present in the MoS2 crystallites, estimated from HREM observations, are active in HDS and that a low number of sulfur vacancies are required.