Concentration profiles through the reaction zone of a low-pressure, slightly lean methane-oxygen-argon flame have been measured, in situ, for the following unstable species: oxygen atoms, hydrogen atoms, hydroxyl radicals, methyl radicals, and formyl radicals. The conditions of measurement, means of identification, and method for estimating the concentrations of the radicals are described. The observed maximum mole fractions are 2.3 X 10-2 for h, 1.1 X 10-2 for oh, and 0.8 X 10-2 for o, the maxima occurring in the recombination zone for these species. The maximum values are in excess of equilibrium concentration by factors of 10-103. Methyl and formyl radicals have maximum mole fractions in the reaction zone of 2.2 X 10-3 and 6 x 10-5, respectively. Equilibrium constants, calculated from the appropriate measured species concentrations for the fastest reactions of the h2 = CO = o2 flame, which is the last stage of this hydrocarbon flame, suggests that these reactions are essentially balanced in this part of the flame.