A detailed mechanistic study of the reaction of [Fe-II(edta)] (edta = ethylenediaminetetraacetate) with molecular oxygen was conducted and the oxidation kinetics were investigated as a function of [Fe-II], [O-2], pH, temperature, and pressure. The observed kinetic data in the presence of an excess of [Fe-II(edta)] can be accounted for in terms of a four-step mechanism: reaction of (1) [Fe-II(edta)H2O](2-) with O-2 by a substitution-controlled process to form [Fe-II(edta)O-2](2-); (II) electron transfer to form an Fe-III-superoxo species; (III) subsequent substitution on a second [Fe-II(edta)H2O](2-) anion followed by electron transfer to give [Fe-III(edta)-O-2(2-)-Fe-III(edta)](4-); and (IV) a fast decomposition yielding the monomeric [Fe-III(edta)](-) and H2O2, which rapidly reacts with [Fe-II(edta)]. Rate and activation parameters for some of these steps are reported and discussed with reference to available literature data.