Ab initio molecular orbital calculations have been performed to investigate the structures and energies of the adduct isomeric radicals arising from the addition reaction of chlorine atom to isoprene. Geometry optimizations of the four isomers of the Cl-isoprene adduct were carried out with density function theory (DFT)-B3LYP and the energies were computed using the single-point calculation of several methods, including MP2, MP4, and CCSD(T). The most energetically favorable isomer was found to be that with Cl addition to the terminal C1-position (isomer I). At the CCSD(T)/6-311G** level of theory, the energies of the other three isomeric radicals relative to I were 1.4, 14.2, and 14.7 kcal mol(-1) for Cl additions to C4- (isomer IV), C3-(isomer III), and C2-positions (isomer II), respectively. The activation energy for Cl migration from the more energetic isomers to the lower energy ones (i.e., from isomers II to I or III to IV) was found to be relatively low (2.8-4.7 kcal mol(-1)), indicating that isomerization of the Cl-isoprene adduct is likely to dominate the chemistry. The present theoretical results are compared to available experimental data based on product analysis for the Cl-isoprene reaction system to infer the atmospheric degradation mechanism of isoprene initiated by chlorine atoms.