Photooxygenation of alpha-methylstyrene with oxygen occurs efficiently in the presence of 10-methylacridinium perchlorate (AcrH(+)ClO(4)(-)) under visible light irradiation in oxygen-saturated acetonitrile (MeCN) to yield acetophenone as the main oxygenated product. No photoinduced oxygenation of alpha-methylstyrene occurs in the absence of AcrH(+) under otherwise the same experimental conditions. Little photodegradation of AcrH(+) occurs in the present photocatalytic system, which provides a clean method of photoinduced oxygenation reaction with molecular oxygen, alternate to the ene reaction of singlet oxygen. The photocatalytic oxygenation of alpha-methylstyrene with oxygen is shown to proceed via photoinduced electron transfer from (x-methylstyrene to the singlet excited state of AcrH(+) ((1)AcrH(+)*) on the basis of the fluorescence quenching of (1)AcrH(+)* by alpha-methylstyrene, the quantum yield determination, and the detection of radical intermediates by laser flash photolysis and ESR measurements.