Due to the implementation of more stringent specifications in sulfur content for diesel oil, a deep understanding of the active phase of Co-Mo/Al2O3 catalysts is necessary to the development of ultra-deep hydrodesulphurization (HYD) catalysts. A series of reductive Co-Mo/Al2O3 catalysts prepared in the lab and the high-active industrial catalyst (G) were studied by in-situ FTIR using CO as probe molecule. The showed a good relationship with the desulphurization activities of the catalysts. With the increase in MoO3 and CoO loading, the desulphurization activity of catalyst increases, and the infrared spectrum changes with the amount of CO adsorbed on the catalyst. There is a new band at 2179 cm(-1) when the MoO3 loading is up to 20% and CoO up to 4. 16%. According to the activities of the catalysts, the appearance of this new band suggests that the catalyst has higher hydrodesulphurization (HYD) activity. Compared with the infrared spectrum of CO on the catalyst of the same MoO3 and CoO loading by the common preparation method, the catalyst prepared with chelating agents has higher HYD activity and its band at 2 179 cm(-1) is stronger. The infrared spectrum of CO adsorbed on optimum Co-Mo/Al2O3 catalyst prepared in the lab is similar to the highly active industrial catalyst (G). Their bands at 2179 cm(-1) are both very strong and their HYD activities are both higher than the others. Thus, the appearance and the increase of the band at 2179 cm(-1) indicate the increase in the HYD activity of Co-Mo/Al2O3 catalysts to some extent, which could be an effective tool for developing ultra-deep HYD catalysts.