In this work, we explored the potential of mesoporous zeolite-supported Go-Mo catalyst for hydrodesulfurization of petroleum resids, atmospheric and vacuum resids at 350-450 degrees C under 6.9 MPa of H-2 pressure. A mesoporous molecular sieve of MCM-41 type was synthesized; which has SiO2/Al2O3 ratio of about 41. MCM-41 supported Co-Mo catalyst was prepared by co-impregnation of Co(NO3)(2). 6H(2)O and (NH4)(6)Mo7O24 followed by calcination and sulfidation. Commercial Al2O3 supported Co-Mo (criterion 344TL) and dispersed ammonium tetrathiomolybdate (ATTM) were also tested for comparison purposes. The results indicated that Co-Mo/MCM-41(H) is active for HDS, but is not as good as commercial Co-Mo/Al2O3 for desulfurization of petroleum resids, It appears that the pore size of the synthesized MCM-41 (28 Angstrom) is not large enough to convert large-sized molecules such as asphaltene present in the petroleum resids. Removing asphaltene from the resid prior to HDS has been found to improve the catalytic activity of Co-Mo/MCM-41(H). The use of ATTM is not as effective as that of Co-Mo catalysts, but is better for conversions of >540 degrees C fraction as compared to noncatalytic runs at 400-450 degrees C.