A series of CoMo and NiMo catalysts supported on TiO2 with various Co/Mo and Ni/Mo molar ratios were prepared to investigate the relationship between the nature of the support and the promotion effect of Co and Ni. The catalytic activities of these catalysts for the hydrodesulfurization (HDS) of dibenzothiophene (DBT) were measured. The HDS activity of CoMo/TiO2 catalysts increased linearly with the addition of cobalt up to Co/Mo ratio of 0.6 and then decreased slightly above this value. Similarly, the HDS activity of NiMo/TiO2 catalysts increased linearly with the addition of nickel up to Ni/Mo ratio of 0.6 and remained almost constant above this value. The sulfur mobility on the CoMo/TiO2 and NiMo/TiO2 catalysts under the reaction conditions was elucidated by a S-35 radioisotope tracer method using S-35 -labeled DBT. The amount of labile sulfur (SO) increased linearly with the addition of cobalt or nickel up to molar ratio of 0.6. In contrast, no significant difference in the rate constants of H2S release (kRE) was observed between Co- or Ni-promoted catalysts and non-promoted Mo catalysts at a given temperature. Therefore, the increase in catalytic activity with the addition of cobalt or nickel can be attributed to an increase in the number of active sites.