The chemical and thermal structure of a premixed rich CH4/air/N-2 flame (phi = 1.18 +/- 0.02) that contains either triphenylphosphine oxide [(C6H5)(3)PO] or hexabromocyclododecane [C12H18Br6] and that is stabilized on a Mache-Hebra burner was studied experimentally using molecular beam mass spectrometry (MBMS) and the microthermocouple technique. Compounds such as hexabromocyclododecane (HBCD) and triphenylphosphine oxide (TPPO) are representative flame-retardant additives that are added to polymers to reduce the flammability of the base polymer. Both compounds provide flame retardation in the gas phase by the production of active species that effectively scavenge key combustion radicals to shut down the combustion process. The MBMS method was used to determine the concentration profiles of stable and active species directly in the flame, which includes atoms as well as free radicals. Thin thermocouples were employed to determine temperature profiles in a flame stabilized on a Mache-Hebra burner at a pressure of I atm. A comparison of the experimental data and simulation results for the flame structure shows that MBMS is suitable for studying the structure of flames that are close to freely propagating conditions. The relative effectiveness of flame inhibition by the compounds tested was estimated from changes in the peak concentrations of H and OH radicals in the flame and from changes in the estimated flame velocity. (C) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.