The thermal decomposition of the phenoxyl radical (1) to form CO plus C5H5., a key reaction in the high-temperature oxidation of benzene, has been studied using ab initio quantum mechanical electronic structure methods. The complete active space (GAS) SCF method was used for geometry optimization of 10 stationary points on the ground-state potential energy reaction surface and computing their harmonic vibrational frequencies. Subsequent calculations using the multireference second-order perturbation theory based on a CASSCF reference function (CASPT2) with 6-31G(d,p) basis set established the energetics along the two alternative reaction paths proposed by Benson and co-workers. The energetics were further corrected for zero point vibrational energy at the CASSCF level of theory. The present study predicts the decomposition of 1 to occur preferably through an electrocyclic cyclization mechanism involving the formation of the 6-oxobicyclo[3.1.0]hex-3-en-2-yl radical (2) as intermediate, rather than by a ring-opening process leading to the (3z)-6-oxo-1,3,5-hexatrien-1-yl radical (4) intermediate. In contrast to early reported experimental kinetic data, the preexponential factor of the thermal Arrhenius expression of the rate constant for the unimolecular decomposition of 1 is predicted to have a normal value (A > 10(13.5) s(-1)).