Hydrodesulfurization (HDS) reactions of radioactive S-35-labeled dibenzothiophene (DBT) were carried out over a series of Ni-promoted Mo/Al2O3 and Co-Mo/Al2O3 catalysts at temperatures between 220 and 280degreesC and under 5.00 MPa. pressure. HDS and hydrogenation (HYD) activity of Mo/Al2O3 catalyst were remarkably enhanced with the addition of nickel, the maximum promotional effect of Ni occurred with a Ni/Mo ratio of ca. 0.5. The amount of labile sulfur (SO) and the release rate constant of [S-35]H2S (k(RE)) increased significantly with the addition of nickel, indicating that nickel makes the sulfur on the catalysts more mobile. The promoting effect of nickel on the activity of Mo/Al2O3 catalyst was attributed to the formation of more active sites. Further, the HDS activity of Co-Mo/Al2O3 catalyst was also enhanced with the addition of nickel; its maximum promotional effect occurred with a Ni/Mo ratio of ca. 0.2 for Ni-Co-Mo catalysts. Ni-promoted Co-Mo/Al2O3 catalysts with lower Ni/Mo ratio had greater S-0 and k(RE) values than Co-Mo/Al2O3 catalyst and resulted in higher HDS activity. This indicates that more active sites were formed with adding Ni to Co-Mo/Al2O3 catalyst, although their numbers were limited. On the other hand, the promoting effect of nickel to Mo/Al2O3 almost disappeared with adding cobalt into Ni-Mo/Al2O3 catalysts, suggesting that cobalt hindered the formation of more active Ni-Mo-S phase or covered the phase.