As a contribution to the development of a process for catalytic upgrading of tarry fuel gases, e.g. coke-oven gas, the conversion of naphthalene, benzene and methane on a nickel catalyst in the presence of H-2 and H2O was studied. The experiments were performed in a tubular flow reactor (total pressure : 1.6 bar; residence time with respect to the empty reactor : 0.3 s; temperature : 400-950 degrees C; and particle diameter of catalyst : 1.5 mm). The kinetic data were obtained by systematic variation of the reaction conditions. At temperatures of more than 800 degrees C, each hydrocarbon is cracked and converted with H2O to CO and H-2. Soot formation does not occur at any temperature. In case of simultaneous conversion of all three hydrocarbons, competitive reactions have to be considered. The rate of chemical reaction on the catalyst is substantially decreased in the presence of H2S. Nevertheless, in a reactor of industrial scale, H2S has only slight influence. The catalyst would be applied with a particle diameter of 19 mm (experiments : 1.5 mm), and the overall reaction rate of hydrocarbon conversion is significantly affected by gas film diffusion.