In order to investigate the relationship between the activating effect of presulfiding and Mo site fine structure in Co-Mo/Al2O3 catalysis, variously pretreated Co-Mo/Al2O3 and Mo/Al2O3 were supplied for hydrodesulfurization (HDS) activity test and Mo K-edge EXAFS (Extended X-ray absorption fine structure) measurement. The HDS activities of Co-Mo/Al2O3 and Mo/Al2O3 pretreated by some combinations of sulfidation (by 5% H2S/H-2) and reduction (by H-2) were examined by HDS reaction of benzothiophene at 270-300 degrees C and 5 MPa in a flow system. The ethylbenzene yield (HDS activity) on Co-Mo/Al2O3 was strongly dependent on the pretreatments as follows : sulfided-reduced > sulfided > reduced-sulfided > reduced CoMo/Al2O3. On the other hand, the ethylbenzene yield on Mo/Al2O3 was very low and hardly dependent on the kind of combination of pretreatments. Mo K-edge EXAFS revealed that the fine structures of Mo site of Co-Mo/Al2O3 and Mo/Al2O3 changed strongly depending on the pretreatment. The fine structure changes of the Mo site of Co-Mo/Al2O3 induced by each pretreatment was almost the same as that of Mo/Al2O3. In both catalysts, MoS2-like structures were formed when the catalysts were initially sulfided, whereas no discrete structure was formed even after post-sulfidation when the catalysts were initially reduced. The presence of Co strongly affects the sensitivity of the catalyst activity to the pretreatment, but does not affect the fine structure of Mo site.