The HDS reaction feature of CCG on Co-Mo/gamma-Al2O3 was investigated in detail to make clear the important factors for the deep HDS of CCG. Two kinds of CCG, which are produced from different origins and contain 229 and 61 ppm sulfur, respectively, were examined. GC-AED analyses showed that both CCG contain 11 alkylthiophenes, 2 alkylbenzothiophenes, 3 alkylthiacyclopentanes, and 2 disulfides. In the reaction at 210 degrees C, 1.6 MPa using a conventional flow reactor of pilot scale, the total sulfur content decreased from 229 to 127 ppm, whereas 22 ppm of thiols was produced. The HDS reactivities of alkylbenzothiophenes were higher than those of alkylthiophenes, and HDS reactivity decreased with increasing the number of alkyl groups. The reactivities of these sulfur compounds in CCG were much lower than those of the corresponding pure compounds obtained by the individual reaction of pure sulfur compounds. The low reactivity of sulfur compounds in CCG was found to be ascribed to the inhibiting effect of olefins in CCG by comparing the HDS reactivity of thiophene in different solvents including olefins. The inhibiting effect of olefin seems to be caused by two types of adsorption of olefin, that is, weak adsorption and strong adsorption.