As a contribution to develop a process for the chemical upgrading of tarry fuel gases, the kinetics of the catalytic conversion of hydrocarbons on a commercial nickel-catalyst in the presence of H-2 and H2O (Sud-Chemie, G 117) were studied. Besides single model hydrocarbons (naphthalene, benzene and methane) and their mixtures, a feed gas obtained by coal pyrolysis was catalytically converted. The experiments were performed in a tubular flow reactor at a total pressure of 160 kPa, a residence time with respect to the empty reactor up to 0.1 s and a particle diameter of 1.5 or 19 mm, varying with the temperature (450-1150 degrees C) and concentrations of H-2, H2O and the hydrocarbons. The influence of H2S and NH3 on the activity of the catalyst was also studied. The results indicate that the Ni-catalyst used is suitable to convert tarry fuel gases into a clean fuel or reduction gas, even if H2S is present. Although the rate of chemical reaction of the hydrocarbons on the Ni-catalyst is substantially reduced by hydrogen sulphide, a rest activity still remains, and all higher hydrocarbons are completely converted to CO, H-2 and CO2 at a temperature of about 1000 degrees C (0.3 vol% H2S; tau = 0.1 s). In contrast to H2S, NH3 has no influence on the conversion of hydrocarbons on the Ni-catalyst, and is just converted to N-2 and H-2. In a reactor of industrial scale, the overall reaction rate of hydrocarbon conversion is significantly affected by gas film diffusion (particle diameter: 19 mm), and is therefore only slightly influenced by H2S.