The thermal retrodiene decomposition of bicyclo[2,2,2]oct-2-ene has been studied in the gas phase in the temperature range 649–718 K. The primary products are ethylene and cyclohexa-1,3-diene which are in equilibrium with the reactant, the equilibrium lying far on the product side under the experimental conditions. The rate constants for the forward reaction were independent of pressure above 0·5 kN m–2(4 Torr) and fitted the Arrhenius equation, log k/s–1= 15·46 – 244,400 J mol–1/2·303RT. At the temperature of the study, the cyclohexa-1,3-diene reacted further, but at an appreciably slower rate than for the primary decomposition. The measured equilibrium constant agrees reasonably with that calculated from thermodynamic data. The probable mechanism of the reaction is discussed and it is concluded that the results favour a concerted rather than a biradical pathway.