Ab initio molecular orbital calculations have been performed to investigate the structures and energetics of the peroxy radicals arising from the OH-initiated oxidation of isoprene. Geometry optimizations of the OH-O-2-isoprene peroxy radicals were performed using density functional theory at the B3LYP/6-31G** level, and individual energies were computed using second-order Moller-Plesset perturbation theory (MP2) and coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations (CCSD(T)). At the CCSD(T)/6-31G* level of theory the zero-point-corrected OH-O-2-isoprene adduct radical energies are 47-53 kcal mol(-1) more stable than the separated OH, O-2, and isoprene reactants. In addition, we find no evidence for an energetic barrier to O-2 addition and have calculated rate constants for the O-2 addition step using canonical variational transition state theory (CVTST) based on Morse potentials to describe the reaction coordinate. These results provide the isomeric branching between the six isoprene-OH-O-2 adduct radicals.