Molybdenum, cobalt-molybdenum and nickel-molybdenum catalysts were prepared over mixed TiO2-Al2O3 oxides containing between 0 and 100% TiO2. The supports and samples were characterized in their oxidic or sulfide state by BET, X-Ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), temperature programmed desorption of ammonia, X-Ray photo electron spectroscopy (XPS) and high resolution electron microscopy (HREM). The three series of catalysts were evaluated in the hydrodesulfurization of thiophene at atmospheric pressure and dibenzothiophene under 45 x 10(5) Pa. The results provided evidence for a support effect on the catalytic activities of the sulfided catalysts. For low or intermediate TiO2 content in the support, a loss of cobalt in the Al2O3 leads to a decrease of activity (compared to the pure Al2O3 supported samples), but for x = TiO2/[TiO2+Al2O3] higher than 0.8, the support effect improved the HDS activity. Moreover, the observed variations in activity depended on the nature of the model molecule used in the catalytic test and/or on the experimental conditions (H-2 or H2S partial pressure). These points are discussed in relation to the mechanisms of the reactions.