Steady-state tracing techniques, using isotopically labeled molecules, were applied to study mechanistic aspects of the carbon and oxygen reaction pathways to form CO over Ni/La2O3 and Ni/Al2O3 catalysts. Over the Ni/La2O3 catalyst, it was found that under steady-state reaction conditions, the quantity of reversibly adsorbed CH4 and the active carbon-containing intermediate species in the carbon pathway to form CO originating from CH4 is higher than the respective quantities derived from the CO2 molecule. Over the Ni/Al2O3 catalyst, much smaller quantities of reversibly adsorbed CH4 and active carbon-containing species, originated from the CH4 molecule, which lead to CO formation were detected. It was also determined that a large quantity of oxygen atoms, originating from the La2O3 support of the Ni/La2O3 catalyst, participate in the reaction scheme. It is concluded that La2O2CO3, which form by the interaction of La2O3 and CO2, may decompose to produce CO or provide oxygen species which react with carbon accumulated on Ni crystallites due to CH4 cracking to produce CO. The latter process is very fast over the Ni/La2O3 catalyst, as compared to carbon accumulation, and this imparts this catalyst its special stability characteristics.