The effects of hydrogen addition on the emission and heat transfer characteristics of oxygen-enriched laminar methane diffusion flames were investigated in a laboratory-scale furnace with a co-axial burner. The volume fraction of hydrogen in the methane-hydrogen blend was varied from 0% to 50%, and the oxygen concentration was varied from 25% to 35%. Results showed that the addition of hydrogen led to an increase in the soot-free length and flame temperature while the degree of increase was less at higher oxygen concentrations. Adding hydrogen chemically enhanced the oxidation of CO to CO2, and this chemical effect was stronger when the oxygen concentration increased. NO emissions increased significantly with the addition of hydrogen, while the rate of this increase decreased with greater oxygen concentrations. The total heat flux increased with the addition of hydrogen, while the rate of this increase was less at higher oxygen concentrations. Although the radiative heat flux increased with higher oxygen concentrations, it did not exceed 6% of the total heat flux at 35% O-2. Moreover, adding hydrogen decreased the radiative heat flux; this decrease was significant at higher oxygen concentrations. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.