Partial oxidation pulse experiments with CH4/O-2 (2/1) and with CD4/O-2 (2/1) have been carried out over the 0.05 and 1.0 wt % Rh/gamma-Al2O3 catalysts to determine the isotope effect of the reaction. On the basis of the results obtained for the isotope effect as a function of temperature and Rh loading, one could conclude that the mechanism of the partial oxidation of methane to synthesis gas is dependent on both metal loading and temperature. It is likely that at low loadings (e.g., 0.05 wt %), the combustion-reforming mechanism is responsible for the reaction, while at high loadings (e.g., 1.0 wt %) a combination of the combustion-reforming and pyrolysis-oxidation mechanisms is responsible at low temperatures (less than or equal to 500 degrees C) and the pyrolysis-oxidation mechanism becomes dominant at high temperatures (less than or equal to 650 degrees C).