A dissociative electron capture process of ethylene oxide yielding the anion radical of the ring-ruptured form, O−–CH2–CH2*, has been demonstrated recently. A similar electron capture process was examined for propylene oxide, 1,2-epoxybutane, and glycidyl methyl ether in an effort to determine which C–O bond would cleave, if any. The epoxy molecules of interest and Na atoms were cocondensed in argon matrices, and the electron transfer between them was effected by mild radiation (λ > 580 nm). ESR examination of the matrices revealed that, in each case, the anion had the ring-opened structure resulting from cleavage of one of the C–O bonds. The external C–O bond cleaved preferentially. A concerted, SN2-type process is suggested between the epoxy molecule and the approaching electron. The conformations of the anion radicals were deduced from the observed α- and β-proton coupling tensors. © 1991, American Chemical Society. All rights reserved.