Hydrodesulfurization (HDS) reactions of dibenzothiophene (DBT) and hydrogenation (HYD) of phenanthrene (PHE) were carried out over several alumina-supported noble metal catalysts at temperatures between 200 degrees C and 400 degrees C and under 50 kg/ cm(2) pressure. A dual noble metal catalyst, Pt(2)-Pd(10), showed similar HDS results and better HYD promotion compared with a conventional Co-Mo catalyst. Further, S-35 radioisotope tracer method showed that almost no labile sulfur was accumulated on Pt/Al2O3 catalyst in the HDS of DBT or HYD of PHE, although this catalyst showed an HDS activity comparable to that of the Go-Mo catalyst. This indicated that the mechanism of DBT HDS on noble catalysts was different from that on the conventional Mo-based catalyst.