The existence of a fast, reversible norcaradiene-cycloheptatriene equilibrium has been demonstrated for 7,7-dicarbometboxycycloheptatriene, synthesized by Berson et al. Measurements of signal intensities and of chemical shifts yield ΔH° = 160 cal/mole and ΔS° = 3 e.u. for the reaction norcaradiene → cycloheptatriene. From lineshape changes between -70 and -140° an activation energy of Ea = 7 kcal/mole and a frequency factor of log A = 11·9 are deduced. © 1969.