Ab initio molecular orbital calculations have been employed to investigate the structures and energetics of the adduct isomers arising from the addition reaction of OH to isoprene. Several levels of ab initio theory were evaluated using a set of organic radical species to establish the appropriate level of approximation. The method of gradient corrected density functionals (NLDFT) in conjunction with moderate basis sets was found to yield satisfactory molecular geometries and vibrational frequencies. Single-point energy calculations were performed using various methods, including MP2, MP4, and CCSD(T). The most energetically favorable isomers are those with OH addition to the terminal carbon positions. At the CCSD(T)/6-311G** level of theory corrected with zero-point energy (ZPE), the isomers with OH additions to isoprene at C1 to C4 positions (i.e., isomers IIV) are 34.8, 24.2, 22.4, and 32.3 kcalmol1 more stable than the OH and isoprene, respectively. The activation energies against OH migration transforming the higher energy isomers into the lower energy ones (i.e., II to I or III to IV) are significant (25.526.5 kcalmol1), indicating that thermal equilibrium of the OHisoprene adduct isomers is unlikely to be established. In addition, we have developed and validated a computationally efficient method to calculate the energetics of the OHisoprene reaction system.