A significant oxidative N-demethylation reaction occurs from a hydroperoxo-copper(II) species when formed within a tripodal tetradentate ligand framework possessing a pendant dimethylamine substrate. This mimics the monoxygenase activity occurring in the copper enzyme PHM. Observation of a product-based alkoxide-Cu intermediate and determination of a reaction kinetic isotope effect (k(H)/k(D(intra)) similar to 2.3) by studying the ligand-N(CH3)(CD3) substrate provide further insights. The mononuclear Cu-II(-OOH) entity or species derived from this can promote biomimetic reactivity and thus requires further attention in biological or synthetic mechanistic studies.