Photocatalytic oxidation of iron(II) complexes by dioxygen occurred using the organic photocatalysts, 9-mesityl-10-methylacridinium ions (Acr⁺-Mes) and 2-phenyl-4-(1-naphthyl) quinolinium ions (QuPh⁺-NA), in the presence of triflic acid in acetonitrile under visible light irradiation. The electron-transfer state of Acr⁺-Mes produced upon photoexcitation oxidized the iron(II) complexes, whereas it reduced dioxygen with protons to produce iron(III) complexes and H₂O₂.