Previous work on carbon-supported hydrodesulfurization (HDS) catalysts has led to the general realization that the nature of the support has a very significant influence on catalytic activity. The present investigation attempts to provide a more focused understanding of the exact nature of this influence. Mostly one support, a commercial carbon black, was subjected to oxidative and/or thermal treatment to modify its surface and structural properties. These were thoroughly examined using temperature-programmed desorption, X-ray diffraction, titrations, and electrophoresis. The various carbon-supported molybdenum catalysts were prepared by equilibrium adsorption and incipient wetness impregnation using four different catalyst precursors. The catalytic activity in thiophene HDS and Fischer-Tropsch synthesis was determined in fixed-bed flow reactors connected on-line to gas chromatographs. The catalysts were characterized by X-ray photoelectron spectroscopy. The heretofore mostly neglected knowledge of carbon surface chemistry was shown to provide the necessary framework for the understanding of the variations in catalytic activity. It is concluded, however, that two conflicting requirements complicate the preparation of highly active (i.e., highly dispersed) molybdenum species on carbon surfaces. On one hand, the introduction of oxygen functional groups provides anchoring sites for catalyst precursor adsorption and thus the potential for its high initial dispersion. On the other hand, this also renders the support surface negatively charged over a wide range of pH conditions. At very low pH conditions, below the isoelectric point of the support, when the attractive forces prevail between the Mo anions and the positively charged carbon surface, Mo polymerization is thought to contribute to catalyst agglomeration. Final catalyst dispersion (i.e., catalytic activity) is also influenced by the thermal stability of the oxygen functional groups on the carbon surface. No significant correlation between structural parameters of the support and catalytic activity was found.