The present study investigated the effects of Co addition on the surface structure and activity of Mo/Al₂O₃ for the hydrogenation of C₆-C₁₀ olefins. CoMo/Al₂O₃ (Co/Mo atomic ratio: 0.56) had lower activity for the hydrogenation of 1-hexene, 1-octene and 1-decene than Mo/Al₂O₃, whereas CoMo/Al₂O₃ had higher activity for the hydrogenation of 2-octene, 2,4,4-trimethyl-2-pentene and cyclohexene. Co/Al₂O₃ showed no activity for the hydrogenation of 1-octene and 2,4,4-trimethyl-2-pentene. NO uptake measurements over Mo/Al₂O₃ suggested that the coordinatively unsaturated sites of Mo atoms located at the edges of MoS₂ clusters were active for the hydrogenation of both 1-octene and 2,4,4-trimethyl-2-pentene. Diffuse reflectance FTIR measurements of NO adsorbed on CoMo/Al₂O₃ (Co/Mo atomic ratio: 0.56) indicated that Co atoms blocked Mo atoms located at the edges of MoS₂ clusters almost completely, i.e. formation of the Co-Mo-S structure. Therefore, formation of the Co-Mo-S structure depressed the activity for hydrogenation of 1-octene. The addition of Co promoter also increased the stacking number of MoS₂ clusters. Presumably the increase in the stacking number of MoS₂ clusters facilitated the adsorption of bulky olefins and promoted the hydrogenation. This may be the main reason for the promoting effect of Co for the hydrogenation of 2,4,4-trimethyl-2-pentene.