A series of hydrodesulphurization (HDS) catalysts containing 12% MoO3 and 2.8% Co3O4 were prepared using γ-Al2O3 modified with various amounts of Li+. The effect of the Li+ cations on the catalytic activity with respect to the HDS of thiophene was studied. A marked decrease in the catalytic activity with increasing Li+ content was found. An explanation based on the Co-Mo bilayer model proposed recently is suggested. This work provides more information about the well-known synergistic effect. Diffuse reflectance spectroscopy and X-ray spectroscopy were used to characterize the specimens.