Abstract In order to give a basis for the utilization of ESR in combustion chemical studies, both diffusion flames and premixed flames of a methane–air mixture have been studied by the flame-in-cavity ESR method. A pair of spectral lines separated by 51.0 mT due to H atoms and a very weak single line due to unidentified reaction intermediates have been recorded from combusting gases in a modified Bunsen burner. The H atoms in the diffusion flame give the spectral linewidth of 0.15 mT and populate a confined region near the luminous cone. In the premixed flame, the H atom concentration is the highest at the outside of the luminous inner cone (reaction zone), but it is distributed as long as 4 cm along the gas stream. The profile of the local concentration has been obtained by the deconvolution method from the observed dependence of the spectral intensity on the location of the flame in the resonant cavity. The maximum concentration can be estimated to be 2.4×1015 atoms/cm3, which is much higher than the equilibrium concentration calculated from the flame temperature. 1200 K, and the oxygen excess ratio, 0.67. The spectral linewidth (0.01–0.04 mT) and the microwave power saturation of the H atoms depend on the location in the flame. These results strongly suggest that the line broadening and the spin relaxation are principally determined by the collision between the H atoms and the molecular oxygen.