Various Co-Mo/Al2O3 catalysts were prepared by deposition of Co(CO)(3)NO either on oxidic or sulfided Mo/Al2O3, The thermo-decomposition step was followed by IR spectroscopy, which showed that cobalt was preferentially deposited on the molybdenum phase. Thiophene hydrodesulfurization (HDS) activity was found to be higher when the precursor was deposited on sulfided Mo/Al2O3, while this catalyst was much more active than that prepared by the classical impregnation method, The nature and the amount of the surface sites of sulfided Co-Mo catalysts were assessed by Fourier transform infrared spectroscopy of adsorbed CO. Unpromoted and promoted coordinatively unsaturated Mo sites were identified on both ex-carbonyl nitrosyl and impregnated catalysts, but the ratio of the two types of sites was found to be strongly dependent on the preparation method. A good correlation was obtained between HDS activity and the integrated area of the IR band characteristic of CO adsorbed on the promoted sites. The high concentration of Co-Mo-S sites on ex-carbonyl nitrosyl catalysts may be related to the low amount of cobalt species on alumina, due to the mode of promoter introduction and to treatments subsequent to cobalt deposition which are milder on ex-carbonyl nitrosyl catalysts than on impregnated ones.