Abstract: The characterization of a series of CoMo/Al₂O₃-SiO₂ catalysts with different SiO₂ contents were studied using in-situ FT-IR, Raman, UV-Vis and H₂-TPR techniques. The results show that the increase of the silica component in CoMo/Al₂O₃-SiO₂ catalysts can promote the transformation of octahedral molybdenum species into tetrahedral molybdenum species, and the higher SiO₂ content can contribute to the transformation. Proper amount of silica component in the mixed supports can alter the interaction between the support and the active metals, resulting in higher hydrodesulfurization(HDS) activity of CoMo/Al₂O₃-SiO₂ catalysts.