Characterization and catalytic activity of tungsten-based hydrotreating catalysts supported on Al2O3(1-x)-TiO2(x) mixed oxides (x = 0.0, 0.1, 0.5, 0.7, 0.95, and 1.0) have been obtained. The supports were characterized, by X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and FT-Raman spectroscopy. The catalysts in their oxided state were characterized by surface area, XRD, UV-Vis diffuse reflectance, FT-Raman spectroscopy, and temperature-programmed reduction (TPR). The catalysts in their sulfided form were characterized by FTIR spectroscopy and high-ri-solution electron microscopy. The catalytic activity of the different catalysts was evaluated in the thiophene hydrodesulfurization reaction at atmospheric pressure. The results indicate that the support composition and properties determine the type of tungsten species present on the surface. The tetrahedral tungsten species present on the alumina support change gradually to different octahedral species as the contents of titania in the support is increased. The presence of titania in the support leads to oxotungstate species with greater lateral interaction than in the case of the pure alumina-supported catalyst. These changes in the type of tungsten species are accompanied by changes in the reducibility and number of species detected by TPR. From the characterization results a picture of the structural changes in the tungsten species as the titania content in the support is increased is proposed, The changes in the catalytic activity trend with titania content can be rationalized in terms of the changes in the dispersion and morphology of the WS2 crystallites, which are in turn related to the type of tungsten species in the oxidic precursors, and also, in the rich titania catalysts (x = 0.95, 1.0), to the effect that a sulfided and/or reduced layer of titania appears to have on the reaction mechanism leading to much higher activities than those expected from purely structural changes of the tungsten species.