A temperature-programmed methane reaction with self de-coking and a fixed-temperature methane reaction were performed over gadolinia-doped ceria (GDC)-supported Ni catalysts. Experimental results reveal that, at below 810 degrees C, CO2 formation rate is higher than that of CO2 where below 700 degrees C, the latter is practically zero. The O species that is needed to form CO and CO, during and after CH4 decomposition are supplied mainly from the bulk lattice of GDC. A drop in the supply rate of the O species from the GDC bulk lattice reduces the rates of CO and CO2 formation; the CO formation rate decreases much more than the CO2 formation rate. The CO and CO2 formation rates can be controlled by both the mobility and the concentration of the bulk lattice oxygen. The concentration of the bulk lattice oxygen influences the self de-coking capability. Low temperature, low concentration of methane gas, or a short methane supply time can result in the formation of only CO2 during self de-coking. (c) 2007 Elsevier B.V. All rights reserved.