In order to better understand combustion processes when large amounts of water (H2O) naturally incorporate into the fuel stream, e.g., the combustion of methane (CH4) hydrates and H2O/fuel emulsions, the extinction limits and structure of counterflow nonpremixed flames of mixtures of H2O vapor and CH4 and air were identified experimentally and computationally. With H2O vapor addition, the maximum flame temperature was experimentally determined, while the flame structure and extinction limits were computed using a detailed kinetic mechanism. Predicted and measured tendencies of the maximum flame temperature for various conditions exhibit encouraging agreement and thus justify using the computational results to analyze the detailed flame structure and determine the extinction limits. The extinction limits (in terms of the H2O to CH4 molar ratio) are reduced with increasing strain rates, implying that flames can sustain more H2O vapor at low strain rates. Thus, the maximum flame temperature at the extinction limits increases with increasing strain rates because there is less H2O to act as a thermal sink. The observed flammable range of the H2O to CH4 molar ratio is comparable to that found in self-sustained combustion of CH4 hydrates. The chemical effects of H2O addition on flame structure are insignificant. (C) 2015 Elsevier Ltd. All rights reserved.