Abstract Hydrodesulfurization (HDS) reactions of the 35S radioisotope-labeled dibenzothiophene (DBT) were carried out over 10% Pd/Al 2O 3 and 2% Pt–10% Pd/Al 2O 3 catalysts. The total amount of the sulfur ( S total) accommodated on the catalysts and the amount of labile sulfur ( S 0) participating in the HDS reaction were determined using a 35S radioisotope tracer method. The sulfided state of noble metal sulfides on two catalysts changed depending on the partial pressure of H 2S in the reaction atmosphere. Taking into account the amount of the sulfur accommodated on the alumina support, the sulfided Pt or Pd species on the catalysts are presented in the form of PdS x or PtS x ( x=0–0.25) in lower partial pressures of H 2S under 5.2 kPa. At the same time, all the sulfur incorporated into the catalyst were almost the labile sulfur. On the other hand, the sulfur further incorporated into the Pd catalyst with further increasing the partial pressure of H 2S in a reaction system over 5.8 kPa, resulting in the formation of the phase of PdS. In contrast, there was no significant change in both S total and S 0 even though the partial pressure of H 2S in the atmosphere increased into ca. 17 kPa in the case of Pt–Pd catalysts. The structures of PdS 0.25 and PtS 0.25 in the Pt–Pd catalyst remained.