The initial isomeric branching in the addition of hydroxyl radical to isoprene controls the distribution of first generation oxidative end products. Two of the four OH-isoprene radical isomers, formed from OH addition to the inner carbons, can undergo a cyclic isomerization pathway to produce α-hydroxy isoprene radicals. This chemical pathway, while well studied for analogous systems, has not been considered previously in the oxidation of isoprene. The hydrogen abstraction reaction is the exclusive pathway in the presence of molecular oxygen for α-hydroxy radicals and results in the formation of C₅ aldehydes or ketones. In this paper the energetics of the isomerization reaction are studied using quantum chemical calculations, and the branching ratios among the isomers are determined using RRKM theory coupled with the master equation formalism.