Highly dispersed MoS2/Al2O3 catalysts containing various amounts Of MoS2 were prepared by the sonochemical decomposition of molybdenum hexacarbonyl in the presence of sulfur and alumina, and tested for the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). The sonochemically synthesized catalysts exhibit an HDS activity about five-fold higher than that of catalysts prepared by A conventional impregnation method over a wide range Of MoS2 loadings. The difference in HDS activity between the two types of catalysts is large, particularly at high loadings Of MoS2, because, in the case of the former catalysts, the activity increases with the amount Of MoS2 to the point where the latter is as high as 25 wt.% while, on the latter catalysts, the activity reaches a saturation level when the amounts of MoS2 are larger than about 15 wt.%. The enhanced HDS activity of the sonochemically synthesized catalysts can be attributed to the improved dispersion of catalytically active species on the support, which is preserved up to high Mo loadings, and the absence of poisoning by sulfur chemisorbed on the catalyst. In both the cases of DBT HDS and 4,6-DMDBT HDS, hydrogenated products are obtained in larger amounts on the sonochemically synthesized catalysts than on the impregnated ones. With respect to the HDS of DBT and 4,6-DMDBT, the latter is enhanced more than the former on the sonochemically synthesized catalysts, which is again due to the high hydrogenation activity of the catalysts.