The microstructure of low-pressure methane-oxygen-argon flames was investigated using modulated molecular beam-mass spectrometry. Profiles of radical and stable species concentration, temperature, and area expansion ratio were used to calculate rate coefficients as a function of temperature for certain elementary reactions occurring in flames. The profiles were modified to simulate the effect of various perturbations and errors possible in sampling and analyses, and the effect on the rate coefficients was discussed. Detailed consideration was given to data reduction techniques, temperature profile composition profile alinement, and the possible temperature dependence of mass spectral fragmentation. The rate coefficients were not dramatically sensitive to the imposed perturbations, although the results depended upon the nature of the reaction in question. Rate coefficients determined for high activation energy reactions and for reactions singularly responsible for the chemical behavior of a given stable species were in agreement with values determined by other techniques.