Photocatalytic oxygenation of 10-methyl-9,10-dihydroacridine (AcrH 2) by dioxygen (O2) with a manganese porphyrin [(P)Mn III: 5,10,15,20-tetrakis-(2,4,6-trimethylphenyl) porphinatomanganese(III) hydroxide [(TMP)MnIII(OH)] (1) or 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinatomanganese(III) acetate [(TPFPP)MnIII(CH3COO)] (2)] occurred to yield 10-methyl-(9,10H)-acridone (Acr -O) in an oxygen-saturated benzonitrile (PhCN) solution under visible light irradiation. The photocatalytic reactivity of (P)MnIII in the presence of O2 is in proportion to concentrations of AcrH2 or O2 with the maximum turnover numbers of 17 and 6 for 1 and 2, respectively. The quantum yield with 1 was determined to be 0.14%. Deuterium kinetic isotope effects (KIEs) were observed with KIE = 22 for 1 and KIE = 6 for 2, indicating that hydrogen-atom transfer from AcrH2 is involved in the rate-determining step of the photocatalytic reaction. Femtosecond transient absorption measurements are consistent with photoexcitation of (P)MnIII, resulting in intersystem crossing from a tripquintet excited state to a tripseptet excited state. A mechanism is proposed where the tripseptet excited state reacts with O 2 to produce a putative (P)MnIV superoxo complex. Hydrogen-atom transfer from AcrH2 to (P)MnIV(O 2•-) generating a hydroperoxo complex (P)Mn IV(OOH) and AcrH• is likely the rate-determining step, in competition with back electron transfer to regenerate the ground state (P)MnIII and O2. The subsequent reductive O-O bond cleavage by AcrH• may occur rapidly inside of the reaction cage to produce (P)MnV(O) and AcrH(OH), followed by the oxidation of AcrH(OH) by (P)MnV(O) to yield Acr -O with regeneration of (P)Mn III.