A molecular beam-mass spectrometer-low pressure flat flame apparatus has been constructed for the purpose of examining the chemical microstructure of normal methane-oxygen-argon flames and those inhibited with halon extinguishants. The composition profiles obtained for normal flames are compared to those of flames inhibited by cf3br. In addition to the unusual stable species (ch4, o2, CO, co2, h2o, h2, and h2co), the inhibitor, its products and intermediates associated with its reactions could be followed. In particular, hydrogen bromide, bromine atoms, hydrogen fluoride, and carbonyl fluoride were observed. The relatively rapid disappearance of inhibitor and early appearance of fluorinated reaction products suggest a short lifetime for the cf3 radical in this syste ob observation of cof2 suggests that cf3br is effective as an inhibitor, at least in part, by virtue of its interference with the oxygen containing species of the normal chain reaction.