Isotopic tracer and nuclear reaction analysis (NRA) are used to probe the identity of oxygen for CO formation during the catalytic partial oxidation (CPOX) of methane to synthesis gas on (18)O(2) labeled Rh (1 wt %)/(Ce(0 56)Zr(0 44))O(2-x) Results reveal that methane is selectively oxidized by lattice oxygen ions from the catalyst to form carbon monoxide. (18)O(2) isotopic exchange experiments, as a function of temperature in the 0-850 degrees C range, were performed on Rh (1 wt %)/(Ce(0 56)Zr(0 44))O(2-x), and (Ce(0 56)Zr(0 44))O(2-x) It was observed that the presence of rhodium considerably accelerates the oxygen exchange with the support, the maximal exchange rates could be observed at lower temperatures, 250 degrees C This may be due to oxygen spillover from the metal particles to the oxide Comparing results from the isotopic exchange experiments on Rh/gamma-alumina and Rh (1 wt %)/(Ce(0 56)Zr(0 44))O(2-x) It was revealed that oxygen conducting materials have a much higher oxygen storage capacity and isotopic exchange rate than non-oxygen conducting materials Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu