The epoxidations of propene and isobutene with peroxyformic acid proceed by a concerted pathway via slightly unsymmetrical transition structures where the differences in the bond distances between the double-bond carbons and the spiro oxygen are only 0.021 and 0.044 A at the QCISD/6-31G* level. In contrast, the more polarizable nature of the carbon−carbon double bond of α,β-unsaturated systems results in an unsymmetrical transition structure for the epoxidation of 1,3-butadiene with an order of magnitude difference in the carbon−oxygen bond distances of 0.305 A at the QCISD/6-31G* level. A highly unsymmetrical transition structure has been also found at this level for the epoxidation of acrylonitrile. Notwithstanding the difference in the extent of asymmetry of the transition structures, both epoxidations of methyl-substituted alkenes and such α,β-unsaturated systems as 1,3-butadiene and acrylonitrile with peroxyformic acid follow a concerted asynchronous pathway. An unsymmetrical transition structure for...