High-performance hydrodesulfurization (HDS) catalysts were prepared by depositing Ni-Mo species over siliceous MCM-41. HDS of dibenzothiophene (DBT), one of the most refractory sulfur-containing molecules in petroleum, was used to evaluate the activity of the prepared catalysts. The Ni-Mo/MCM-41 showed better performance than the Co-Mo/MCM-41 in the HDS of DBT, probably due to the enhanced hydrogenation ability of Ni-Mo sulfides. The selectivities of the products indicated that both hydrogenolysis and hydrogenation followed by desulfurization play important roles in HDS of DBT, in contrast with the results of HDS of DBT catalyzed by Co-Mo/MCM-41. The optimum HDS activity was observed at a Ni/Mo atomic ratio of 0.75, similar to that of Co-Mo/MCM-41 catalysts but higher than the conventional gamma-Al2O3 -supported catalysts. The higher optimal Ni/Mo ratio may be attributed to the higher dispersion of the active species and to generation of more Ni-Mo pairs on the surface of MCM-41. The high performance of the prepared Ni-Mo(0.75)MCM-41 catalyst was confirmed by the HDS of 4-methyldibenzothiophene, 4,6-dimethyldibenzothiophenc, and a high-sulfur diesel. The S-35 isotope tracer investigation revealed that sulfur atoms accommodated on the surface after HDS could be released only by the introduction of a sulfur-containing compound, indicating that sulfur-atom exchange between sulfur-containing molecules and the active sites is involved in the HDS reaction. Accordingly, a reaction mechanism for HDS is proposed.