An experimental and numerical investigation of flame temperature and intermediate radical (OH, O, H, CH) concentrations in hydrogen (H-2)-natural gas (NG) hybrid fuel diffusion flames at a burner exit Reynolds number 150 are presented. Radial concentration profiles of temperature and radicals in the near-burner region (8 mm above the burner tip) for different mixtures of hydrogen-NG (100% NG, 80-20% NG-H-2, 50-50% NG-H-2, 20-80% NG-H-2, 100% H-2 by volume) are reported. Both experimental and computational findings show that as a result of increased fuel burning, the stoichiometric contour moves inward with the increase of hydrogen content in the fuel mixture. The 100% hydrogen flame temperature is close to its equilibrium value, while due to the effect of radiation the mixed fuels have lower temperatures than their equilibrium values. The OH radical concentration profile shows a peak in the stoichiometric contour. Within experimental uncertainties the measured and the computed values of OH radical concentration agree well. The OH concentration increases with the increase of hydrogen concentration in the mixture. The radial profiles of O atom concentration have a peak in the lean side of the fuel oxidizer interface. The photolytic production of O atom affects the measured values. The maximum H atom concentration occurs in the rich side of the stoichiometric contour. The computed peak concentration of H radical for 100% NG flame is comparable with its equilibrium concentration, which indicates the existence of partial equilibrium of H in hydrocarbon flames. The maximum production of CH radical occurs in the rich side of the flame surface. Compared to the experimental results, computation predicts a lower CH concentration because of the use of reaction mechanism without C-2 chemistry. The CH radical concentration decreases with the increase of hydrogen content in the mixed fuel. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.