Se-S exchange properties of hydrodeselenium (HDSe) reaction of selenophene on Co-Mo sulfide catalysts prepared by a CVD method were investigated on the basis of the amount of exchangeable sulfur atoms per active site and Co K-edge XAFS. The Co K-edge EXAFS for the CVD-Co/MoS₂ catalysts after HDSe reaction was analyzed by a curve fitting method. EXAFS analysis revealed a relationship between the coordination numbers of Co-S and Co-Se shells and the number of incorporated selenium atoms per Co (Se/Co). The change of the coordination numbers of Co-S and Co-Se shells during the reaction are compatible with the dinuclear Co-Mo-S model previously proposed from EXAFS analysis and magnetic properties. A linear relationship was observed between the number of exchanged sulfur atoms per Co atom (H₂S/Co) and the activity for thiophene HDS reaction, indicating that the HDS activity depends on the amount of exchangeable sulfur atoms in Co-Mo sulfide catalysts. It was also found that the amount of maximum exchangeable sulfur depends on the catalyst support and sulfidation conditions.