Exposure of matrix-isolated acetyl benzoyl peroxide to UV light (λ > 2800 Å) produces methyl benzoate, the methyl-phenyl radical pair, and carbon dioxide. The results of the low-temperature irradiation reveal that methyl benzoate is formed by recombination of the benzoyloxy-methyl radical pair. Since phenyl acetate was not detected during the low-temperature irradiation, it was concluded that the lifetime of the acetoxy-phenyl radical pair is too short to allow concentrations that could be detected by infrared spectroscopy. Ab initio calculations are used to provide an explanation for the dramatic lifetime differences between the benzoyloxy and acetoxy radicals. In addition, it was shown that electronic excitation of the phenyl radical does not induce a chemical reaction with carbon dioxide. Arguments based on experiment and theory are presented to explain this unexpected result. © 1983 American Chemical Society.