The effect of Pt and Ru (similar to 0.6%) added to Mo/Al2O3 sulfide catalyst on the simultaneous hydrodesulfurization (HDS) of thiophene and hydrogenation (HYD) of cyclohexene was studied. The addition of Pt and Ru significantly increased the HDS and HYD activities of the Mo/Al2O3 catalyst. The addition of Pt led to a weaker synergy in HDS, which was almost independent of the reaction temperature. The addition of Ru led to much stronger synergy in both reactions, which increased at higher reaction temperatures. The synergy in HDS over Ru-promoted catalyst occurred between 240 and 320 degrees C, approaching values of up to 6. The strong synergy in HYD (up to 4) appeared only at temperatures 300-330 degrees C and was accompanied by a more than twofold increase in the apparent activation energy. As the most probable explanation, a change in the quality of the sites participating in HYD was proposed. HYD mainly proceeds on separate Ru and Mo sulfides at lower temperatures and while on mixed Ru-Mo-S sites at above 300 degrees C. This change is ascribed to a partial release of Ru-Mo-S sites by desorption of thiophene or H2S at the higher temperatures, which compete for the adsorption and reaction with HYD. (c) 2005 Elsevier Inc. All rights reserved.