The inhibiting effect of olefin on CCG HDS (found in our previous report) was further investigated hom a mechanistic point of view. A small amount of thiophene (corresponding ed ca. 100 Mrt ppm of sulfur content) and varying amounts of olefins (diisobutylene and 1-octene) dissolved in toluene were hydrotreated on Co-Mo/Al2O3 at 150 degrees C at 1.3 MPa. Both the total HDS (thiophene HDS minus thiol and sulfide formation) and thiophene HDS were strongly inhibited by a small amount of olefin (I mol %). However, when the olefin content was increased over 20 mol %, thiophene HDS rather recovered while the total HDS monotonously decreased. Both isoolefin and n-olefin can interact with the thiophene HDS active site, resulting in the inhibition on thiophene HDS. The interaction between the HDS active site and olefins was further studied by examining the effect of H2S, an well-known inhibitor for HDS, on hydrogenation of olefins. The hydrogenation of n-olefin as well as thiophene HDS was sensitively inhibited by H2S, while the hydrogenation of 2,4,4-trimethyl-2-pentene was promoted. The interaction was also studied by examining the effects of Co, an well-known promoter for HDS, on HDS and olefin reaction over different Co/(Co + Mo) ratio catalysts. HDS was promoted by Co; however, isoolefin hydrogenation was retarded slightly and n-olefin hydrogenation was largely retarded. Three types of active sites for thiophene HDS, n-olefin hydrogenation, and isoolefin hydrogenation, were proposed.