MacrOc-yclic metal complexes,and p-benzoquinones are commonly used as co-catalytic redox mediators in aerobic oxidation reactions, In an effort to gain insight into the mechanism and energetic efficiency of these reactions, we investigated Co(salophen.)-catalyzed aerobic oxidation of p-hydroquinone. Kinetic and spectroscopic data suggest that the catalyst resting-state consists of an equilibrium between a Coll(salophen) complex, a Co-III-superoxide adduct, and a hydrogen-bonded adduct between the hydro quinone and the Co-III-O-2 species. The kinetic data, together with density functional theory computational results, reveal that the turnover-limiting step involves proton-coupled electron transfer from a semi-hydroquinone species and a Co-III-hydroperoxide intermediate. Additional experimental and computational data suggest that a coordinated H2O2 intermediate oxidizes a second equivalent of hydroquinone Collectively, the results show how Co(salophen) and p-hydroquinone operate synergistically to mediate O-2 reduction and generate the reactive p-benzoquinone co-catalyst.