Abstract-Data are presented on the structure of propane diffusion flames burning in a concentric stream of air, with and without the addition of diluents (carbon dioxide and nitrogen) to the fuel jet and air stream. The flame length and the fraction of heat release radiated are documented as functions of diluent concentrations in fuel jet and air stream. The effects of diluent addition on the temperature profiles, composition profiles showing species CO2, H2O, CO, N2, O2, C3H8,NO and pyrolysis species of C3H8 and axial profiles of radiant power emitted are also presented. The results show that the diluents reduce flame length, stability, radiation and concentrations of nitric oxide in the flames. The effects of diluents on the flame structure can be attributed primarily to the changes in concentrations of various species rather than to the changes in temperature. The problems of using flue gas recirculation in large combustion systems are indicated.