The hydrotreating CoMo/gamma-Al2O3 catalysts (12 wt% Mo and 0.7-5.0 wt% Co) have been prepared by impregnation. The CoMo catalysts obtained from Anderson heteropolyoxomolybdate (NH4)(3)[Co(OH)(6)Mo6O18]center dot 7H(2)O has been investigated by nitrogen physisorption, XRD, UV-Vis DRS, FT-IR, and TPR. The results are discussed in relation to the catalytic activity for the thiophene and 1-benzothiophene reactions. The influence of Co content on both the phase composition and catalytic activity in the HDS reactions has been shown. The effect of Co on the activity in the HDS reactions depends on its previously loaded amount. It is the most pronounced with the catalyst having Co/Mo ratio equal to 0.27.