Photooxidation of 1,3-diphenylisobenzofuran (DPBF) in aromatic solvents at 30° gives a monomeric peroxide along with minor amounts of o-benzylphenyl benzoate, whereas in CCl4o-dibenzoylbenzene is the principal reaction product. 1,3-Diphenylisobenzofuran is very susceptible to autoxidation at this temperature (kp/(2kt)1/2 = 4 M−1/2 s−1/2, kp ∼ 105 M−1s−1 2kt ∼ 5 × 108 M−1s−1) yielding a polyperoxide (50%) and o-dibenzoylbenzene (35%). Clearly, in the absence of appropriate control experiments, neither rapid disappearance of DPBF nor production of o-dibenzoylbenzene are sufficient to prove the intermediacy of singlet delta oxygen during molecular oxygen oxidations.