Photoirradiation of the absorption band of the 10-methylacridinium ion (AcrH(+)) with visible light in deaerated CH3CN/H2O (1:1 v/v) containing pivalic acid ((BuCOOH)-C-t) and less than 1 equiv of NaOH results in the selective formation of 9-tert-butyl-9,10-dihydro-10-methylacridine (AcrHBu(t)). The same product is obtained in O-2-saturated CH3CN/H2O under visible light irradiation. Photoirradiation of the absorption band of AcrHBu(t) with UV light in deaerated CH3CN/H2O (1:1 v/v) results in the formation of tert-butyl hydroperoxide ((BuOOH)-O-t), accompanied by regeneration of AcrH(+). This cycle can be repeated several times. When AcrH(+) is replaced by the 9-phenyl derivative (AcrPh(+)), AcrPh(+) acts as an effective photocatalyst for the one-pot photooxygenation of (BuCOOH)-C-t in the presence of less than 1 equiv of NaOH relative to (BuCOOH)-C-t with 02 to yield (BuOOH)-O-t and (BuOH)-O-t. The photocatalytic oxygenation mechanism is discussed based on the detection of radical intermediates by laser flash photolysis and ESR measurements as well as quantum yield determination.