The oxidation of NH3 during oxy-fuel combustion of methane, i.e., at high [CO2], has been studied in a flow reactor. The experiments covered stoichiometries ranging from fuel rich to very fuel lean and temperatures from 973 to 1773 K. The results have been interpreted in terms of an updated detailed chemical kinetic model. A high CO2 level enhanced formation of NO under reducing conditions while it inhibited NO under stoichiometric and lean conditions. The detailed chemical kinetic model captured fairly well all the experimental trends. According to the present study, the enhanced CO concentrations and alteration in the amount and partitioning of O/H radicals, rather than direct reactions between N-radicals and CO2, are responsible for the effect of a high CO2 concentration on ammonia conversion. When CO2 is present as a bulk gas, formation of NO is facilitated by the increased OH/H ratio. Besides, the high CO levels enhance HNCO formation through NH2 + CO. However, reactions NH2 + O to form HNO and NH2 + H to form NH are inhibited due to the reduced concentration of O and H radicals. Instead reactions of NH2 with species from the hydrocarbon/methylamine pool preserve reactive nitrogen as reduced species. These reactions reduce the NH2 availability to form NO by other pathways like via HNO or NH and increase the probability of forming N-2 instead of NO. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.