O-18 Tracer study of perfluorinated poly(p-phenylene)-catalyzed photohydroxylation of benzene to phenol with (H2O)-O-18 and O-16(2) revealed that the hydroxylation initially occurs by the attack of the hydroxyl radical (HO.)-O-18 generated by the photooxidation of (H2O)-O-18, but the hydroxyl radical (HO.)-O-16 derived from the concurrently produced (H2O2)-O-16 comes to participate in further hydroxylation as the photocatalysis proceeds.