The atmospheric chemistry of benzene oxide/oxepin, a possible intermediate in the atmospheric oxidation of aromatic hydrocarbons, has been investigated in a large volume photoreactor at 298 K and atmospheric pressure using in situ FTIR spectroscopy for the analysis. Rate coefficients of (10.0±0.4)×10 ^-11 and (9.2±0.3)×10 ^-12 cm ³ molecule ^-1 s ^-1 have been determined for the reaction of benzene oxide/oxepin with OH and NO ₃ radicals, respectively. Reaction with OH radicals produces almost exclusively the (E,Z)- and (E,E)-isomers of hexa-2,4-dienedial, whereas reaction with NO ₃ produces (Z,Z)-hexa-2,4-dienedial and unidentified organic nitrates. Phenol has been observed as a major product of the thermal decomposition, visible and UV photolysis of benzene oxide/oxepin. The results are discussed in conjunction with the oxidation mechanisms of aromatic hydrocarbons. The major atmospheric sinks of benzene oxide/oxepin will be reaction with OH radicals and photolysis and, under smog chamber conditions with high NO ₂ concentrations, also reaction with NO ₃ .