A mesoporous aluminosilicate molecular sieve with MCM-41 type structure was synthesized using aluminum isopropoxide as the Al source. Supported Co-Mo/MCM-41 catalysts were prepared by co-impregnation of Co(NO3)(2). 6H(2)O and (NH4)(6)Mo7O24 followed by calcination and sulfidation. For comparison, conventional Al2O3-supported sulfided Co-Mo catalysts were also prepared using the same procedure. These two types of catalysts were examined at two different metal loading levels in hydrodesulfurization of a model fuel containing 3.5 wt% sulfur as dibenzothiophene in n-tridecane. At 350-375 degrees C under higher H-2 pressure (6.9 MPa), sulfided Co-Mo/MCM-41 catalysts show higher hydrogenation and hydrocracking activities at both normal and high metal loading levels, whereas Co-Mo/Al2O3 catalysts show higher selectivity to desulfurization. Co-Mo/MCM-41 catalyst at high metal loading level is substantially more active than the Co-Mo/Al2O3 catalysts.