Dioxygen (O(2)) activation by iron enzymes is responsible for many metabolically important transformations in biology. Often a high-valent iron oxo oxidant is proposed to form upon O(2) activation at a mononuclear nonheme iron center, presumably via intervening iron superoxo and iron peroxo species. While iron(IV) oxo intermediates have been trapped and characterized in enzymes and models, less is known of the putative iron(III) superoxo species. Utilizing a synthetic model for the 2-oxoglutarate-dependent monoiron enzymes, [(Tp(iPr2))Fe(II)(O(2)CC(O)CH(3))], we have obtained indirect evidence for the formation of the putative iron(III) superoxo species, which can undergo one-electron reduction, hydrogen-atom transfer, or conversion to an iron(IV) oxo species, depending on the reaction conditions. These results demonstrate the various roles that the iron(III) superoxo species can play in the course of O(2) activation at a nonheme iron center.