This work deals with the reburn chemistry in oxy-fuel combustion of methane. It was studied experimentally in an atmospheric-pressure flow reactor under diluted conditions in N-2 and CO2, respectively. The experiments covered temperatures from 1173 to 1773 K and stoichiometries ranging from fuel-rich to fuel-lean. The results showed similar NO reduction efficiencies in N-2 and CO2 under reducing conditions. while under stoichiometric and fuel-lean conditions. the NO reduction obtained in CO2 was higher than that obtained in N-2. The temperature at which CO exhibited its maximum concentration coincided with a sharp increase in NO reduction. This temperature was higher in the experiments with CO2 compared to those in N-2. The results were interpreted in terms of an updated detailed chemical kinetic model. The effect of CO2 is to increase the OH/H ratio, lower the overall concentration of the O/H radical pool, and increase the availability of CO. Under reducing conditions, the different ways that CO2 interacts with the nitrogen chemistry largely cancel oil and the overall impact is small. Under stoichiometric conditions, the NO reduction is enhanced in CO2 because of the suppression of the O/H radical pool. ill particular of atomic oxygen.