Oxygen-18 scrambling in carbonyl-O-18-labelled cyclopropanecarbonyl peroxide (CPO) and benzoyl cyclopropanecarbonyl peroxide has been suggested to take place via the recombination of intimate acyloxyl radical pair, based on the following experimental observations, (i) The rate of thermal decomposition of CPO decreased with:the increase in the viscosity of hydrocarbon solvent; however the rate of the O-18-scramblnig of CPO in Nujol, a highly viscous solvent, was found tp be twice that in hexane, Examination of the effect of solvent viscosity on the rate of the decomposition of CPO by Pryor-Neuman's equation reveals that the O-18-scrambling in CPO takes place almost solely through recombination of the intimate cyclopropanecarboxyl radical pair, In octane, the rate constant for dissociation of the intimate cyclopropanecarboxyl radical pair was estimated to be one order of magnitude greater than those for recombination and decarboxylation of the intimate acyloxyl radical pair. (ii) The enthalpy of activation for the decomposition of CPO was found to be 2.0-2.8 kcal mol(-1) greater than that for the O-18-scrambling of CPO in CCl4 which corresponds to the energy barrier for dissociation of the intimate cyclopropanecarboxyl radical pair in the solvent cage, The entropy of activation for O-18-scrambling was found to be small but positive in contrast to the large negative values of entropies of activation for usual concerted sigmatropic rearrangements, (iii) Successive substitution of the cyclopropyl groups of CPO with phenyl groups reduced markedly the rate of the O-18-scrambling in the peroxides, This substituent effect is the same as that in the initial bond breaking type of the Cope rearrangement of homotropylidenes; however, opposite to that is the Cope rearrangements of open chain hexa-1,5-dienes in which-a substantial extent of the new bond has been formed in the transition state.