The effects of radiative transfer on the structure and extinction limits of counterflow H-2/O-2/N-2 diffusion flames are studied numerically using detailed kinetics and transport properties. The radiative properties of the main emitting species, H2O and OH in these flames, are represented using a statistical narrow-band model. The radiative transfer equation and flow governing equations are solved in a coupled manner. The model is first validated by comparing numerical results with Rayleigh temperature and total flame radiative emission measurements. It is then applied to the numerical study of radiative effects on flame structure and properties. These effects, i.e., a decrease in flame temperature, flame width and production of minor species, are found to be the most important for high values of inlet H-2 mass fraction and for low strain rates. Quantitative values of radiative low strain rate extinction limits are given. The limits of validity and the discrepancies due to the optically thin medium approximation are also investigated.