The increased instability of CO2 (or exhaust gas) recycled oxy-fuel flames is generally attributed to the increased specific heat of the gaseous mixture resulting in lower flame temperatures. In the present paper, we study the role of CO2 addition through exhaust gas recirculation on the combustion chemistry of an oxy-methane flame. A flame structure analysis of various combinations of methane-oxidant flames has been carried out using the GRI 3.0 325-step mechanism. Further, comparative reaction rate analysis has been carried out under nitrogen and CO2-dilutions to bring out the oxygen leakage in the two systems. These studies indicate that the presence of CO2 in the oxidant has a profound effect on the reaction mechanism principally through two reactions, one involving the H radical and another involving the HO2 radical, which become predominant in oxy-fuel combustion and contribute to the excessive oxygen leakage. (C) 2011 Elsevier Ltd. All rights reserved.