DFT and ab initio molecular orbital calculations have been performed to investigate the structures and energetics of the Cl-O-2-isoprene peroxy radicals arising from the Cl-initiated oxidation of isoprene. Geometry optimizations of the chloroalkenylperoxy radicals were performed using density function theory (B3LYP), and the energies were computed with the single-point calculation using different levels of theory for electron correlation and basis set effects. At the CCSD(T)/6-31G(d) level of theory corrected with zero-point energy (ZPE), the chloroalkenylperoxy radicals are about 39 to 43 kcal mol(-1) more stable than the separated reactants (i.e.,O-2 + Cl + isoprene). 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. The results provide the isomeric branching between the Six Cl-O-2-isoprene peroxy radicals, indicating that the two beta-chloroalkenylperoxy radicals with initial Cl addition at Cl and C4 positions and subsequent O-2 addition at C2 and C3 positions,respectively, play an important role in determining the reaction pathways and final product distributions of the Cl-isoprene reaction system.