Evaluation of steam-added methane-hydrogen blends through the comparison of flame structures and pollutant emissions is numerically conducted with detailed chemistry. The composition of fuel is systematically changed from pure methane and pure hydrogen to the blending fuel of methane-hydrogen-steam through the molar addition of H2O. Flame structure is changed considerably for CH4-H-2 flames and CH4-H-2-H2O flames in comparison to pure methane flame. The discernible differences in major flame structure are compared among a pure methane flame, CH4-H-2 flames and CH4-H-2-H2O flames. Especially, emission indices of CO increase and then decrease after showing a maximum in the increase of methane mole fraction for CH4-H-2 flames and in the increase of H2O mole fraction for CH4-H-2-H2O flames, while those Of CO2 increase monotonously. These behaviors are mainly caused by the competition of the production through the reaction step HCO + H2O -> H + CO + H2O with the destruction of CO by the reaction step CO + OH -> CO2 + H. The changes of thermal NO and Fenimore NO are also analyzed for various combinations of the fuel composition. Importantly, contributing reaction steps to thermal NO and Fenimore NO are addressed in pure methane, CH4-H-2 flames and CH4-H-2-H2O flames. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.