Mononuclear copper(II) superoxo complexes have attracted much recent attention due to their strong relevance to the reactive intermediate involved in the catalytic reactions of copper monooxygenases [1].These enzymes catalyze hydroxylation reaction of aliphatic substrates by molecular oxygen at the simple mononuclear copper reaction center. To date, both side-on and end-on superoxo copper(II) model complexes have been reported to provide important insights into the structure and physicochemical properties of the reactive intermediate involved in the copper monooxygenases. In this study, reactivity of mononuclear copper(II) end-on superoxo complexes supported by a tridentate and tetradentate ligands have been investigated in order to shed light on the dioxygen activation mechanism at the mononuclear copper reaction centers.