UV-vis spectral titrations of a manganese(III) corrolazine complex [Mn-III(TBP(8)Cz)] with HOTf in benzonitrile (PhCN) indicate mono- and diprotonation of Mn-III(TBP(8)Cz) to give Mn-III(OTf)(TBP(8)Cz(H)) and [Mn-III(OTf)(H2O)(TBP(8)Cz(H)(2))][OTf] with protonation constants of 9.0 x 10(6) and 4.7 X 10(3) M-1, respectively. The protonated sites of Mn-III(OTf)(TBP(8)Cz(H)) and [Mn-III(OTf(H2O)(TBP(8)Cz(H)2)][OTf] were identified by X-ray crystal structures of the mono- and diprotonated complexes. In the presence of HOTf, the monoprotonated manganese(III) corrolazine complex [Mn-III(OTf)(TBP(8)Cz-(H))] acts as an efficient photocatalytic catalyst for the oxidation of hexamethylbenzene and thioanisole by O-2 to the corresponding alcohol and sulfoxide with 563 and 902 TON, respectively. Femtosecond laser flash photolysis measurements of Mn-III(OTF)(TBP(8)Cz(H)) and [Mn-III(OTf)(H2O)(TBP(8)Cz(H)(2))][OTf] in the presence of O-2 revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state of Mn-III(OTF)(TBP(8)Cz(H)) reacted with O-2 with a diffusion -limited rate constant to produce the putative Mn-IV(O-2(center dot-)(OTf)(TBP(8)Cz(H)), whereas the tripseptet excited state of [Mn-III(OTf)(H2O)(TBP(8)Cz(H)(2))][OTf] exhibited no reactivity toward O-2. In the presence of HOTf, Mn-v(O)(TBP(8)Cz) can oxidize not only HMB but also mesitylene to the corresponding alcohols, accompanied by regeneration of Mn-III(OTf)(TBP(8)Cz(H)). This thermal reaction was examined for a kinetic isotope effect, and essentially no KIE (1.1) was observed for the oxidation of mesitylene-d(12), suggesting a proton-coupled electron transfer (PCET) mechanism is operative in this case. Thus, the monoprotonated manganese(III) corrolazine complex, Mn-III(OTf)(TBP(8)Cz(H)), acts as an efficient photocatalyst for the oxidation of HMB by O-2 to the alcohol.