The reactions with methane and ethane of hot chlorine atoms recoiling from the 40Ar (γ, p)39Cl nuclear reaction have been studied. Displacement reactions, analogous to those of hot hydrogen with alkanes, were found. In the case of methane, these gave CH339Cl and CH239Cl. Yields of these molecules show little pressure dependence, providing negative evidence for unimolecular decomposition due to internal excitation of primary products. A kinetic theory analysis of moderator data indicates that fast chlorine atoms are relatively less reactive than is hydrogen. Furthermore, energy transfer in inelastic collisions with methane appears to be relatively ineffective. The indicated weak coupling between translational and vibrational modes appears due to poor matching between the mean tune of collision and the C-H bond vibration period. This study appears to provide the first definite evidence for collisional decomposition of hot molecules. CH339Cl as formed by reaction of 39Cl with CH4 must be translationally excited. A kinetic theory analysis shows that this hot molecule has a considerable probability of breaking up on striking argon atoms. The likelihood of decomposition on collision with methane is, as expected, much less.