The photooxidation of several substituted isobenzofurans has been studied. Kinetic behavior at low substrate concentrations shows that a second mechanism other than that involving singlet oxygen is operative, and the absence of azulene quenching demonstrates that the second mechanism involves direct addition of oxygen to excited singlet isobenzofuran. Rate constants for this singlet substrate-ground state oxygen reaction are in the range of 10 M_1 sec-1 and the mechanism is calculated to contribute about 10% of the photooxidation at high isobenzofuran concentration, ranging upward to virtually 100% at lower substrate concentrations. The mechanism is suggested to be via charge-transfer rather than concerted addition and the possibility of such an oxidation mechanism is generalized to other aromatic systems.A great deal of evidence has been amassed in the past few years to indicate that photoinduced addition of molecular oxygen to several types of substrates proceeds primarily if not exclusively via excited oxygen molecules in the (^g) electronic state.2