Abstract: The active sites of reduced Mo/γ-Al₂O₃ and Co-Mo/γ-Al₂O₃ hydrotreating catalysts were studied by in-situ FT-IR using CO and NO as probe molecules; the results were correlated with their activity in hydrodesulfurization (HDS) and behavior in temperature programmed reduction (H₂-TPR). The results indicate that there exist three main adsorption sites on the surface of Co-Mo/γ-Al₂O₃ catalysts. The addition of cobalt to Mo/γ-Al₂O₃ is able to modify Mo sites and to introduce active Co sites; an increase of the molybdenum content may also increase the number of active sites and improve the catalytic activity of Co-Mo/γ-Al₂O₃. FT-IR spectra of CO and NO co-adsorption is able to distinguish the spectra overlapping of different active sites, which reveals that CO and NO are actually adsorbed on different Mo active sites.