The partial oxidation of CH4/O-2-and CH4/air-mixtures into synthesis gas on a Ni/MgO/Al2O3-catalyst in an adiabatic fixed bed reactor was modelled. Diffusional limitations on the various reactions were accounted for through effectiveness factors, averaged over the operating conditions and calculated from a number of off line pellet simulations. The evolution of the coke content of the catalyst with time was included in the simulation so as to check whether or not a steady state reactor operation was possible. The influence of the feed composition (addition of steam and carbon dioxide) and the operating conditions on the coke content was also investigated. For constant methane flow rate and constant total pressure the partial pressure of oxygen decreases when steam is added. This leads to a higher coke content of the catalyst, while coke deposition occurs in a larger area of the reactor, since oxygen is a stronger oxidant than steam.