Abstract The rate of reaction of CH 3 H has been measured at 300 K over the pressure range 50–1000 Torr with Ar and SF 6 as diluent gases. Flash photolysis of azomethane/ethylene mixtures was employed, coupled with gas chromatographic analysis and numerical simulation of the product yields. The rate data were compared with a RRKM analysis, using a minimum density of state criterion for locating the activated complex. The limiting high pressure rate constant so obtained is 1.5 ± 0.7) × 10 −10 cm 3 molecule −1s −1. The RRKM model was found to give high temperature, high pressure rate constant for the dissociation of CH 4 which are significantly higher than those found experimentally.