The effect of pressure on the methyl radical combination reaction was measured following the pressure dependence of the cross-combination reaction for methyl–ethyl or methyl–isopropyl pairs of radicals. These were produced by the photolysis of the corresponding azo-compounds. The method was found to be very suitable for this type of study and ‘fall-off’ curves at 15, 60, and 100° were determined. In the range of pressures studied (from a few torr to 0·005 torr) the kuni/k∞ ratio varied from 1·0 to 0·25. The results do not support a loose activated complex model for the methyl radical combination reaction.