A CoMo hydrotreating catalyst was presulfided ex situ by impregnation of the ditertio-nonylpentasulfide (TNPS) and subsequent hydrogen treatment up to 623 K at Various pressures (1-21 bar). The genesis of the sulfide phase at each step of the activation procedure, impregnation, drying, hydrogen treatment was studied using several physicochemical techniques, UV-Vis Reflectance spectroscopy, XPS and HREM. By comparison with the conventional sulfiding procedure (H-2-H2S), this ex situ sulfidation process brings about a significant increase in the dispersion of the active molybdenum sulfide phase. In agreement with these results, the hydrodesulfurization activity in the thiophene conversion is also increased.