The solvent kinetic hydrogen isotope effect showed that proton transfer is at least partially rate determining for the acetolysis of cyclopropanes which span a range of 1010 in reactivity. The energies and structures of protonated cyclobutanes were calculated and provide an explanation for the large difference in reactivity between cyclopropanes and cyclobutanes despite their similarity in enthalpies of reaction. The rates and products of acetolysis of a series of alkyl-substituted cyclopropanes were examined. The data, along with the results of ab initio calculations, indicate that for alkyl-substituted cyclopropanes, the protonated species is highly unsymmetrical. Cleavage of the cyclopropane ring always occurs so that the nucleophile becomes attached to the most substituted carbon, but the proton may attack either of the remaining carbons. Proton attack may lead to either retention or inversion of configuration depending on the orientation of the attacking proton with respect to the ring.