Abstract The oxidation reactions of phenyl and 1-naphthyl radicals through reaction with O 2 were investigated through a quantum chemical molecular orbital method as potential sources of PAH species containing 5-membered rings in hydrocarbon flames. The reaction between phenyl radical and O 2 was seen to produce phenoxy radical in a single elementary step. The same mechanism was followed in the reaction between 1-naphthyl radical and O 2 with formation of naphthoxy radical in a single step. In both phenyl and 1-naphthyl oxidation reactions, the variation of energy and atomic arrangement with the reaction coordinate was found to be similar and their rate coefficients, evaluated through transition state theory, to agree with each other. Thus, because the oxidation reaction of a species composed of one aromatic ring (phenyl radical) and that of a species composed of two aromatic rings (1-naphthyl radical) proceed through the same mechanism and have similar rate coefficients, the oxidation of species composed of an arbitrary number of aromatic rings can be supposed to proceed through that mechanism and have the same rate coefficient. The mechanism of the subsequent decomposition of phenoxy radical into cyclopentadienyl radical and CO agreed with the mechanism reported in the literature, and the decomposition of naphthoxy radical was found to be similar to that of phenoxy radical. In summary, the routes C 6 H 5+ O 2→ C 6 H 5 O+ O→ C 5 H 5+ CO+ O C 10 H 7+ O 2→ C 10 H 7 O+ O→ C 9 H 7+ CO+ O, where C 5H 5 and C 9H 7 are, respectively, cyclopentadienyl radical and indenyl radical, proceed through the same mechanism. From these results it is suggested that the formation of species containing a peripheral 5-membered ring (aryl-5) through partial oxidation of a peripheral 6-membered ring of arbitrary aryl radicals (aryl-6), described generally as aryl-6+ O 2→ aryl-6- O+ O→ aryl-5+ CO+ O, can be thought to proceed through the same mechanism. The rate coefficients of the first step are the same for any aryl-6 species, but they were found to be much smaller than values reported in the literature for phenyl as aryl-6 species.