The thermal isomerisation of trans-1-cyclopropylbuta-1,3-diene to 3-vinylcyclopentene has been studied in the gas phase. In the temperature range 271–316° the isomerisation is quantitative, homogeneous, and obeys first-order kinetics. The rate constants obtained at various temperatures in this range fit the Arrhenius equation k= 1013·36 ± 0·10 exp(–44,470 ± 260/RT) sec.–1. The reaction is almost certainly unimolecular. By comparison of the energy of activation for this reaction with those obtained for other substituted cyclopropanes it is possible to obtain an estimate of the acyclic pentadienyl radical resonance energy, on the assumption of a biradical mechanism, of 21·7 ± 2 kcal. mole–1.