We have performed an exhaustive theoretical study, using a density functional theory (DFT) and ab initio techniques, of the possible isomers of the OH-toluene-O 2 radical. DFT calculations of the all electron type using the hybrid B3LYP approach and 6-31G * orbital basis set were employed. In addition to the well-established ortho position, addition of OH at C 1 on the benzene ring of toluene was also considered for the initial methylhydroxycyclohexadienyl adduct. In all, 28 different intermediate structures of the OH-toluene-O 2 system, consisting of peroxyl radicals, bicyclic structures, and epoxides, have been explored through fully optimized electronic structure calculations. Starting from the 1,3-O 2 -methylorthohydroxycyclohexadienyl radical, or ortho-OH adduct, several peroxyl radicals are found to have low-lying structures contained within a small energy range (about 1 kcal/mol). Only two bicyclic structures are stable with respect to the methylhydroxycyclohexadienyl radical plus O 2 , one of them being clearly favored. The four possible epoxy structures are all found to lie more than 15 kcal/mol lower than any of their peroxyl and bicyclic isomers. The preference, first noted by Bartolotti and Edney, for structures in which the OH group lies on the same side of the ring as the O 2 group, is obeyed in all cases. If the 1-CH 3 , 1-OH cyclohexadienyl radical (or C 1 -OH adduct) is used as the initial adduct, three peroxyl radicals are expected to be formed, while two bicyclic structure and three epoxides need to be considered. These structures are found to be, in general, less stable than the ones arising from the ortho adduct. However,