The effects of the sulfidation atmosphere on the local structure and intrinsic activity of the active sites, Co-Mo-S, are studied with SiO2-supported Co-Mo sulfide catalysts prepared by a chemical vapor deposition method using Co(CO)(3)NO. The Co-Mo sulfide catalysts were sulfided in a stream of 10% H2S/H-2 or 10% H2S/He or their combinations. The intrinsic activity Of Co-Mo/SiO2 presulfided in H2S/He is 1.6 times as high as that of Co-Mo-S Type II presulfided in H2S/H-2 for the hydrodesulfurization (HDS) of thiophene, demonstrating that the location and/or local structure of Co-Mo-S are varied by the sulfidation atmosphere as well as by the MoS2-support interaction. The catalysts were characterized by X-ray absorption fine structure and transmission electron microscope. It is suggested that the intrinsic activities of Co-Mo-S on the S-edge and Mo-edge Of MoS2 particles are distinctly different for HDS. Extended X-ray absorption fine structure analysis shows a good correlation between the coordination number of Co-S and the intrinsic activity of "active Co-Mo-S". (C) 2009 Elsevier Inc. All rights reserved.