CoMo/ZrO2-SiO2 mixed oxide supported catalysts have been investigated for hydrotreating of model molecules in a micro-reactor at 400 degreesC and atmospheric pressure. Activity studies were carried out for thiophene hydrodesulfurization (HDS) and cyclohexene hydrogenation (HYD). These activities on sulfided catalysts were evaluated as a function of support composition [Zr/(Zr + Si) = 0.15, 0.30, 0.45, 0.70], rnolybdenum (2-14 wt.%) and promoters Co, Ni (1-5 wt.%) contents. The supported catalysts were characterized by BET specific surface area, XRD. UV-DRS and low temperature O-2 chemisorptions. Low temperature O-2 chemisorption (LTOC) at -78 degreesC was conducted on the sulfided catalysts in order to determine Mo dispersion on the surface of the support and concentration of coordinative unsaturated sites (CUS) which is related to the HDS and HYD activities. The oxygen uptakes and catalytic activities results followed similar trend with molybdenum variation indicating that the number of active sites for HDS varies with Mo content. HDS and HYD activities increase up to 6 wt.% Mo and beyond this loading activities decrease with further increase Mo content. The characterization results of XRD, UV-DRS and LTOC agreed well about the Mo dispersion. Incorporation of ZrO2 in SiO2 improves weak interaction of active phases with the SiO2 support and overcomes poor dispersion on the support surface. Therefore, ZrO2 counter part plays a key role to provide better activity through enhanced number of active sites as well as activity per site. The promotional effect increases with decreasing SiO2 content in the support composition. (C) 2004 Elsevier B.V. All rights reserved.