Benzoquinone decreases the rate of dicyanoanthracene-sensitized photooxidation of trans-stilbene, apparently by intercepting the reactant 02-. We have reported the electron transfer initiated oxidation of trans-stilbene (TS) sensitized by the singlet excited state of dicyanoanthracene (DCA).lp2 We proposed that oxidation products were formed by reaction of TS+ with 02-(Scheme 1). The characteristic absorption spectrum of TS+ was observed in these reactions by laser flash photolysis. 2 The presence of 02-, formed by oxidation of DCA-by 3O 2, was not directly established, but was inferred from the exothermicity of the electron, transfer from DCA-to O2 (0.7 kcal/mole)' and the rapid disappearance of DCA-absorbance in the flash system in the presence of 02.2 Scheme 1: 1 DCA + TS -> DCA' + TS+ 2C6H5CH0 The intermediacy of 02-in these reactions remains questionable. Several other oxygenation reactions of olefins (01) are known to proceed via an olefin radical cation 314; in most of these reactions, 02-is not present, so that 302 must be the oxygen source; a chain process is often the major reaction path: 01 [oX3 ,01: -+01.0,< O2 po1*02 + 01t