The thiophene hydrodesulfurization activities of Ni and Ni---Mo sulfide catalysts supported on activated carbon were measured at atmospheric pressure and the catalyst structures were studied by means of X-ray photoelectron spectroscopy, dynamic oxygen chemisorption, and chemical sulfur analysis. The Ni/C catalysts contained a Ni3S2-like phase which, due to the impregnation method, was considerably enriched in the outer shell of the carbon support grains. Both for the Ni/C and Ni---Mo/C catalysts no correlation was observed between the dynamic oxygen chemisorption and the catalytic activity and for the Ni/C catalysts indications for full oxidation of the Ni3S2 particles were obtained. The specific catalytic activity per Ni atom was lower in the Ni/C catalysts than in the Ni---Mo/C catalysts. Since the local Ni structure in these catalysts is different, however, it proved not possible to decide whether the Ni atoms in Ni---Mo/C catalysts are the actual active sites or not.