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A commercial CoMo/Al2O3 catalyst was subjected to one of several pretreatments which involved a sequence of sulfidation and reduction, and the catalyst activity was subsequently examined for the 10MPa in a flow system. Ethylbenzene (EB) and 2,3-dihydrobenzothiophene (DHBT) were selectively produced.The conversion and product composition depended on the catalyst pretreatment. The catalyst activity for hydrodesulfurization (HDS) decreased in the following manner: sulfided>sulfided-then-(HGN) on the pretreatment was different from that of HDS. The gas phase addition of 5vol% H2O did not change the total conversion but inhibited HDS almost completely not only on the sulfided catalyst but also on the reduced catalyst, while the amount of DHBT was apparently increased by the addition of H2S.Comparing the present results with the effects of the pretreatment and H2S on the hydrodenitrogenation (HDN) activity previously reported, the order of HDS activity in the absence of H2S was parallel to that of HDN activity in the presence of H2S. On the basis of this parallelism, the relation between HDS active sites and HDN active sites was discussed.