Using a Fabry-Perot interferometer, the Doppler widths of lines of the CH bands emitted from the reaction zone of an oxy-acetylene flame have been examined. The experimental arrangement and method of computation are described. The effective translational temperatures of the excited CH radicals are around 4000 degrees K in the low-pressure flames, compared with theoretical maximum flame temperatures around 2700 degrees K. At atmospheric pressure the translational temperature is near the expected flame temperature. The effective rotational temperatures have also been measured for the CH bands 4315 and 3900 angstrom and do not differ much from the theoretical maximum flame temperatures for any of the several flames examined. The method of excitation of the CH is discussed. It seems likely that the radicals are excited by collision with other active species in the flame; the high translational temperature shows that we do not have normal thermal excitation, but there are also difficulties in attributing the effects to true chemiluminescence. For the weak CH band at 3143 angstrom the effective rotational temperature is very high, especially at low pressure.