Photocatalytic oxygenation of 10-methyl-9,10-dihydroacrid ine (AcrH(2)) by dioxygen (O-2) with a manganese porphyrin [(P)Mn-III: 5,10,15,20-tetrakis-(2,4,6-trimethylphenyl)-porphinatomanganese(III) hydroxide [(TMP)Mn-III(OH)] (1) or 5,10,15,20-tetrakis(pentafluorophenypporphyrinatomanganese- (III) acetate [(TPFPP))Mn-III(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)Mn-III in the presence of O-2 is in proportion to concentrations of AcrH(2) or O-2 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 KEE = 22 for 1 and KIE = 6 for 2, indicating that hydrogen-atom transfer from AcrH(2) is involved in the rate-determining step of the photocatalytic reaction. Femtosecond transient absorption measurements are consistent with photoexcitation of (P)Mn81, 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)Mn-IV superoxo complex. Hydrogen-atom transfer from AcrH(2) to (P)Mn-IV(O-2(center dot-)) generating a hydroperoxo complex (P)Mn-IV(OOH) and AcrH(center dot) is likely the rate-determining step, in competition with back electron transfer to regenerate the ground state (P)Mn-III and O-2. The subsequent reductive O-O bond cleavage by AcrH(center dot) may occur rapidly inside of the reaction cage to produce (P)Mn-v(O) and AcrH(OH), followed by the oxidation of AcrH(OH) by (P)Mn-V (O) to yield Acr=O with regeneration of (P)Mn-III.