Hydrodesulfurization (HDS) reactions of the S-35 radioisotope-labeled dibenzothiophene (DBT) were carried out over 10% Pd/Al2O3 and 2% Pt-10% Pd/Al2O3 catalysts. The total amount of the sulfur (S-total) accommodated on the catalysts and the amount of labile sulfur (SO) 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 H2S 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 PdSx or PtSx (x = 0-0.25) in lower partial pressures of H2S 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 H2S 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 Stotal and So even though the partial pressure of H2S in the atmosphere increased into ca. 17 kPa in the case of Pt-Pd catalysts. The structures of PdS0.25 and PtS0.25 in the Pt-Pd catalyst remained. (C) 2003 Elsevier Inc. All rights reserved.