The title reaction involves a strong interplay between dissociation and isomerization. A potential energy surface is drawn as a function of the two most important internal coordinates (i.e. the CH bond stretching and the 1,2 motion of a hydrogen atom shifting from its original position to the other carbon atom) with the remaining coordinates adjusted to their optimized positions. The low energy reaction path which connects the ethylenic structure to the lowest asymptote (relative to the non-classical bridged C 2 H + 3 fragment) is controlled by a saddle point. RRKM calculations have been carried out in four different approximations with progressive inclusion of anharmonicity and overall rotation. The entropy of activation indicates a fairly loose character for the transition state.