The catalytic activity of nickel, a trace metal indigenous to residuum used in coprocessing of coal and petroleum residuum, was investigated in hydrogenation and heteroatom removal reactions using model systems at coprocessing conditions. Hydrocarbons and heteroatomic species of S, N, and O having similar chemical structures to some compounds present in coal and residuum were used as models. The active Ni catalyst was generatedin situby thermal decomposition of organic nickel complexes, all of which were active for hydrogenation of the models selected except for nickel naphthenate. The active Ni promoted ring saturation in aromatic and heteroatomic compounds rather than ring hydrogenolysis or heteroatom removal. In the presence of either elemental sulfur or organic sulfur compounds, the catalytic activity of Ni for hydrogenation and heteroatom removal was eliminated. Because of the high sulfur content in coal and residuum, inherent catalytic activity from the nickel indigenous to residuum for hydrogenation and heteroatom removal reactions during coprocessing may be hindered.