A series of Mo catalysts supported on TiO2 with various surface areas were prepared. In hydrodesulfurization (HDS) of dibenzothiophene (DBT) the activities of the series of catalysts were estimated. The results show that the activities of the series of catalysts increased linearly with the surface areas of TiO2 supports. The sulfur mobility on the sulfided Mo/TiO2 catalysts under the reaction conditions was elucidated by a S-35 radioisotope tracer method using S-35-labeled DBT. The results suggest that the sulfur exchange rate constants (k(RE)) were approximately the same for all catalysts at a given temperature and that the amount of labile sulfur (SO) increases linearly with the surface area of TiO2 supports. The sulfidation states of the series of catalysts were also investigated by a S-35 radioisotope pulse tracer method using S-35-labeled H2S. The results indicate that in spite of the coverage of Mo species on TiO2 support, the sulfidation of TiO2 support would still occur at 300 C. The sulfided catalysts could be reduced under H-2 atmosphere.