Rate -ratio asymptotic (RRA) analysis is carried out to elucidate the influence of carbon monoxide on the structure and critical conditions of extinction of nonpremixed methane flames. Steady, axisymmetric, laminar flow of two counterflowing streams toward a stagnation plane is considered. One stream, called the fuel stream is made up of a mixture of methane (CH4) and nitrogen (N-2). The other stream, called the oxidizer stream, is a mixture of oxygen (O-2), and N-2. Carbon monoxide (CO) is added either to the oxidizer stream or to the fuel stream. Chemical reactions, represented by four global steps, are presumed to take place in a thin reaction zone. To the leading order the reactants, CH4, O-2, and CO are completely consumed in the reaction zone. On either side of this thin reaction zone, the flow field is inert These inert regions represent the outer structure of the flame. The outer structures provide matching conditions required for predicting the structure of the reaction zone. In the reaction zone, chemical reactions are presumed to take place in two layers the inner layer and the oxidation layer. The scalar dissipation rate at extinction is predicted from results of the asymptotic analysis and compared with previous measurements and computational predictions using detailed chemistry. The predictions of the asymptotic analysis are found to agree well with the experimental data for CO addition to the fuel stream, and for small amounts of CO addition to the oxidizer stream. For large amounts of CO addition to the oxidizer stream, the approximations introduced in the asymptotic analysis become inaccurate. A key finding is that with increasing amounts of CO added to the oxidizer stream the scalar dissipation rate at extinction first increases and then decreases. It is attributed to changes in location of the inner layer within the reaction zone. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.