A comparison of the activity behaviors of the mechanistically similar reactions of carbon dioxide reforming and steam reforming of methane was carried out at 450 similar to 500 degrees C over nickel catalysts with samaria- and gadolinia-doped ceria and alpha-alumina as the supports. Results show that the activity behaviors of carbon dioxide reforming and steam reforming of methane are similar and very sensitive to the oxygen-vacancy properties of the support, with a drastic increase of the activities as the temperature increases from 450 to 500 degrees C. Nevertheless, a difference in the activity behaviors between carbon dioxide and steam reforming has been observed and is due to a difference in the reaction mechanisms of CO2 and H2O dissociations. Possible carbon deposition (coking) is lessened due to the reaction of the surface carbon species with the lattice oxygen or the surface O species as produced from CO2 or H2O. It was found that the samaria-doped ceria supported nickel catalyst has better de-coking ability than that of the gadolinia-doped ceria supported one. An oxygen-transport reaction mechanism for doped-ceria supported Ni catalyst has been proposed and shown to explain the activity behaviors successfully.