Anaerobic reaction of alpha- or beta-alkylcobinamides (alpha- or beta-RCbi(+)’s) with the corresponding alkyl free radical, R(.) (where R = CH3, CH3CH2, or CH3CH2OCH2CH2), generated by the Fenton reaction using Fe2+ and an alkyl hydroperoxide, RC(CH3)(2)OOH, causes isomerization and leads to mixtures of alpha- and beta-RCbi(+)’s. The reaction does not occur, however, under aerobic conditions or under anaerobic conditions in the presence of an excess of the free radical scavenger H-Tempo. In addition, alpha-CH(3)CH(2)Cbi(+) reacts with 50 molar equiv of tert-butyl hydroperoxide and Fe2+ to give a mixture of alkylcobinamides that contains 6% alpha-CH(3)Cbi(+) and 94% beta-CH(3)Cbi(+), showing that multiple transalkylations occur. A Co-II-induced isomerization and the S(H)2 mechanism are ruled out on the basis of the known reactivity of RCbi(+) and product analysis. A mechanism is proposed which involves a direct oxidative free radical displacement by an R(.) to the metal of RCbi(+) via a dialkylcobalt(IV) corrinoid species. Since the reaction leads to equilibration of the two diastereomers under mild conditions, it can be used to study the equilibria between diastereomeric RCbi(+)’s. Thus, the equilibrium for the diastereomeric ethyl-13-epicobinamides, in which the e propionamide side chain of the corrin ring has been epimerized from the alpha to the beta face of the corrinoid, lies significantly more toward the a diastereomer than that for the normal ethylcobinamides. This represents the most direct experimental evidence obtained to date that the corrin ring side chains control the relative steric accessibility of the two faces of the cobalt corrinoids.