AbstractMethane is a primary product of pyrolysis of 3‐chloro‐l‐butene at temperatures in the range 776–835°K, and from its rate of formation values have been obtained for the limiting high‐pressure rate constant of the reactionThese may be represented by the expression log [(k1)∞/sec−1] = (16.7 ± 0.3) − (71.5 ± 1.5)/θ, where θ = 2.303RTkcal/mole. Assuming a zero activation energy for the reverse reaction and that over the experimental temperature range the rates at which a methyl radical adds on to chlorobutene are comparable to those at which it abstracts hydrogen, the activation energy for the dissociation reaction leads to a value of 83.2 ± 1.9 ckal/mole forD(HCHClCH:CH2) at 298°K. TakingD(HCHClCH2CH3) = 95.2 ± 1.0 kcal/mole a value of 12.0 ± 2.1 kcal/mole is obtained for the resonance energy of the chloroallyl radical. This value in conjunction with resonance energies obtained in earlier work indicates that substitution of a hydrogen atom on the carbon atom adjacent to the double bond in the allyl radical leads to no significant variation in