The rate constants for rearrangement of alpha-substituted cyclopropylcarbinyl radicals have been measured by nitroxide radical-trapping (NPT). Those bearing methyl, dimethyl, or cyclopropyl substituents undergo ring opening 2-3 times more slowly than does cyclopropylmethyl radical, but the reaction is essentially irreversible under the conditions used. Phenyl and tert-butoxycarbonyl alpha-substituents retard the rate of ring opening more strongly and enhance the rate of ring closure of the corresponding substituted but-3-enyl radicals. Thus for c-C(3)H(5)CHPh at 60 degrees C, k(ring open) = 5.4 x 10(5) s(-1), k(ring close) = 1.5 x 10(7) s(-1), and the equilibrium favors the ring closed form (K-equil = 0.04). The implications of the possible reversibility of the ring opening of substituted cyclopropylcarbinyl radicals for cyclopropane probe studies of metal hydride reduction and other chemical/biochemical reactions are assessed. Most of the cyclopropylcarbinyl radicals were generated from tert-butyl peroxyglyoxaletes (ROC(O)CO(3)Bu(t)) via alkoxycarbonyl radicals (ROCO). This method allowed the determination of the rate constants for decarboxylation of ROCO when R is t-Bu, PhCH(2), c-C(3)H(5)CMe(2), c-C(3)H(5)CHMe, (c-C3H5)(2)CH, or c-C3H5CHC6H(5).