The present investigation attempts to provide a better understanding of the influence of the nature of the carbon support on the HDS activity of Mo, Ni, and NiMo catalysts. For this purpose a high purity activated carbon was subjected to oxidative treatments with HNO3 to modify its surface properties. NiMo catalysts supported on the resulting activated carbons were prepared and characterized by TPR, XRD, and SEM-EDX, and their activity for HDS of thiophene at 30 bars and 375 degrees C was evaluated. The results obtained showed that oxidation of the carbon surface does not affect the HDS activity and other characteristics of the supported Mo phase. In contrast, the HDS activity of the Ni catalysts is enhanced by acid treatments of the carbon support. In this case, introduction of oxygen-containing functional groups (O-(S)) leads to a strong interaction of O-(S)-Ni during impregnation, which becomes essential to achieving and preserving high nickel dispersion. This effect on Ni dispersion is also reflected on the HDS activity of the bimetallic NiMo/C catalysts, The synergistic effect of the bimetallic catalysts is observed only when oxygen functional groups are present on the carbon surface, which are necessary for a good HDS activity, mainly because they enhance Ni-Mo interactions that produce the highly active Ni-Mo-S phase, A NiMoO4-like phase formed during impregnation seems to be the precursor for the active sulfide phase over the present NiMo/C catalysts.