A study of the kinetic isotope effect (CH4/CO2 --> CD4/CO2) for carbon dioxide reforming of methane to synthesis gas shows that an isotope effect exists with k(CH4)/k(CD4) ratio of 1.05-1.97, depending on reaction temperature and catalyst applied. The attainment of stable performance over Ni/La2O3 catalyst is found to be related to the strong chemisorption of CO2, weak chemisorption of CH4 and slow rate of CHx formation, and fast rate for CHx removal by oxidation.