Pre-reforming of natural gas has been studied on a nickel catalyst at 480-550 degrees C and 20 bar using a TEOM (Tapered Element oscillating Microbalance) reactor. The focus has been on carbon formation and the main objective was to study the influence of C-2-C-3 hydrocarbons in the methane feed on carbon deposition on the catalyst. Coking thresholds for different mixtures of hydrocarbons were determined by varying the steam to carbon (S/C) ratio at,various temperatures. The steady-state coking rate decreases with increasing S/C ratio, and increases with increasing carbon number of the hydrocarbon. Unsaturated hydrocarbons show a strong effect on coking rates and on carbon thresholds. For mixtures of methane and propane/aropene, the steady-state coking rate as well as the coking threshold decreased with a decrease in the mole fraction of propane/propene and with an increase in the hydrogen mole fraction. The coking rate revealed a complicated temperature dependency, and a minimum in the coking rate and in the coking threshold was detected at 500 degrees C. A relationship between the critical steam to carbon ratio and the thermodynamic carbon activity is also developed based on a suggested reaction mechanism, which can properly predict the role of hydrocarbon, hydrogen and water in carbon formation. (c) 2004 Elsevier B.V. All rights reserved.