Carbon dioxide reforming of methane was studied over Ni/yttria-doped ceria (YDC) with various nickel loadings and calcination temperatures of YDC to seek insights into the activation Of CO2 and CH4, as well as the activity and deactivation behaviors of the catalyst. Compared with that of Ni/gamma-Al2O3, the rate of Ni/YDC is enhanced due to the synergistic effect of nickel and surface oxygen vacancies of YDC. Both nickel and YDC may participate in the activation Of CO2 and CH4. Nickel is apt to dissociatively adsorb CH4 to form CHx and adsorb CO to result in CO, whereas YDC is capable of adsorbing and decomposing CO) into CO and O. Catalysts prepared with YDC calcined at higher temperatures exhibit higher activities of CO formation. However, due to larger crystallite size and less surface oxygen vacancies of these YDC supports, the oxidation rate of carbon precursors on the catalyst surface is reduced, thereby causing these catalysts more susceptible to be deactivated. The deactivation of Ni/YDC is attributed to the deposition of inactive carbon on the catalyst surface, and the dissociative adsorption of methane is primarily responsible for the coking deposition. The concentration of CO2 is important for the activity and lifetime of the catalyst. The oxidation of carbon precursors on the catalyst surface may be expedited by increasing the concentration of CO2 thereby enhancing the coking resistivity of the catalyst. (C) 2003 Elsevier Science B.V. All rights reserved.