The epoxidation of alkenes by peroxyl radicals in the gas phase is examined, and it is demonstrated that the activation energies for 36 epoxidation reactions between 17 alkenes and 5 peroxyl radicals correlate well to the charge transfer, or the corresponding energy decrease, in forming the peroxyalkyl adduct, or the difference between the ionization energy of the alkene and the electron affinity of the peroxyl radical. These correlations have been used to estimate five previously unmeasured epoxidation rate constants relevant to propene autoxidation.