The structures and energetics of 14 geometric isomers of C6H502, including phenyl peroxy (C6H5OO), hydroperoxy phenyl (C6H4OOH), seven-membered ring C6(O)H5O isomer, three isomers of hydroxyl phenoxy [C6H4O-(OH)], seven isomers of OC6H5O (including the three with epoxy structures), and a stable dual-ring C6H5-O2 π-complex have been calculated by ab initio molecular orbital methods. Geometries have been optimized at the UHF/6-31G* level, and their relative energies with respect to C6H5 + O2 have been refined by the spin-projected UMP3(PUMP3) method with UHF/6-31G* zero-point energy corrections. All C6H5O2 isomers identified except w-OC6H5O, which is not a local minimum, are more stable than the C6H5 + 02 reactants. The most stable isomers are the three C6H4O-(OH) radicals, which are 105-110 kcal/mol below C6H5 + O2. They are followed by the seven-membered ring C6-(O)H5O, the m-epoxy isomer, other C6H5O radicals, and the initial adduct C6H5OO, which is 41 kcal/mol lower than C6H5 + O2. The ir-complex, formed by the association of C6H5 with O2 at the center of the O═O bond, is 27 kcal/mol below the reactants. The C6H4OOH radical, which may be formed by intramolecular H-abstraction, is more stable than C6H5 + O2 by 9 kcal/mol. The results suggest that several mechanisms may be involved in the C6H5 + O2 reaction. Vibrational frequencies determined for each isomer at the UHF/6-31G* level of theory have been discussed.