The hydrodesulfurization (HDS) activities of sulfided TiO2 with various surface areas were characterized under the typical HDS reaction conditions. The results showed that HDS activities increased linearly with increasing surface area of TiO2, suggesting that the surface sulfide TiS2 is the active phase of the HDS reaction on sulfided TiO2 catalyst. The sulfidation states of TiO2 with various surface areas were investigated using a S-35 radioisotope pulse tracer method. The results showed that the Ti atoms on the surface of TiO2 could be completely sulfided to TiS2 at 400 degreesC, whereas the sulfidation of Ti atoms in the bulk of TiO2 would occur slowly at 500 degreesC. When the sulfided TiO2 was reduced in H-2 at 400 degreesC, TiS2 was reduced to the sulfidation state TiS1.5, leading to the formation of Ti3+ ions and sulfur anion vacancies. Moreover, HDS reaction mechanisms on sulfided TiO2 catalysts under the reaction conditions were elucidated by a 35S radioisotope tracer method using S-35-labeled dibenzothiophene (DBT). The results showed that less labile sulfur was detected on the sulfided TiO2 catalysts, suggesting that the HDS mechanism on TiO2 catalysts was very different from that on Mo-based catalysts.