CoO-MoO3/MgO and NiO-MoO3/MgO catalysts were prepared from MoO3/MgO by impregnation with methanol solutions of Co nitrate and Ni nitrate, respectively, followed by calcination at 350degreesC. The texture of the starting MoO3/MgO was preserved during this preparation. The hydrodesulfurization activity of the pre-sulfided catalysts was evaluated using the reaction of benzothiophene (1.6 MPa, 300-360degreesC). Commercial CoMo/Al2O3 (Shell 344) and NiMo/Al2O3 (Shell 324) catalysts were used as reference points of activity. A strong synergistic effects in activity between Co(Ni) and Mo in Co(Ni)Mo/MgO catalysts was observed with an optimum Co(Ni)O content of about 3-4 wt%. The most active Co(Ni)Mo/MgO catalysts were 1.5 to 2.3 times more active than their corresponding Al2O3-supported counterparts. MgO should be considered a new promising support for Co(Ni)Mo sulfide catalysts. However, its sensitivity to water and propensity, to form solid solutions with CoO and NiO must be taken into consideration in catalyst preparation.