Photooxidation of 9-substituted 10-methyl-9,10-dihydroacridine (AcrHR) with oxygen occurs efficiently in the presence of 9,10-dicyanoanthracene (DCA) and scandium triflate [Sc(OTf)(3)] under visible light irradiation in oxygen-saturated acetonitrile (MeCN) to yield the 9-substituted 10-methylacridinium ion (AcrR(+)) and 11,02 or the 10-methylacridinium ion (AcrH(+)) and the oxygenated products of R such as ROOH, depending on the type of substitutent R. No DCA-photosensitized oxidation of AcrHR occurs in the absence of Sc3+ under otherwise the same experimental conditions. The observed selectivities for the C(9)-H vs C(9)-C bond cleavage of AcrHR in the DCA-photosensitized oxidation of AcrHR in the presence of Sc(OTf)(3) agree with those for the cleavage of radical cations of AcrHR (AcrHR(.+)) depending on the type of substituent R. Such product selectivities, being consistent with the electron-transfer oxidation of AcrHR, combined with quantum yield determination, the O-1(2) phosphorescence decay dynamics, and the detection of radical ion intermediates in the laser-flash photolysis experiments reveal the electron-transfer radical chain mechanism for the DCA-photosensitized oxidation of AcrHR initiated by photoinduced electron transfer from AcrHR to the singlet excited state of DCA.