Much enhanced acid catalysis was observed in oxygen atom transfer (OAT) reactions by a mononuclear nonheme Cr(III)-superoxo complex, [(Cl)(TMC)Cr III (O 2 )] + (1), in the presence of triflic acid. In the acid-catalyzed reactions, the reactivity of 1 in OAT of thioanisole was enhanced significantly, showing more than 10 4 -fold acceleration in rate. Electron transfer (ET) from electron donors to 1 also occurred only in the presence of HOTf. The enhanced reactivity of 1 by HOTf was explained by proton-coupled electron transfer from electron donors, such as ferrocene, to 1 in light of the Marcus theory of ET. The present study reports for the first time the dramatic proton effect on the chemical properties of metal-superoxo species.