The kinetics of the reaction of oxygen with aqueous Fe II (EDTA) solutions has been determined in a range of process conditions (C Fe II (EDTA) ) 15-60 mol/m 3 , P O2 ) 5-20 kPa, pH ) 5-8, T ) 298-328 K, C NaCl ) 0-15 kg/m 3 ) using a gas-liquid stirred cell reactor. The oxygen absorption rates were modeled using an expression derived by De Coursey using the penetration theory of mass transfer. Within the range of process conditions applied, the reaction was shown to be first order in oxygen and second order in iron chelate. The temperature dependence of the kinetic constant may be expressed as (pH ) 7, no salt addition): k 12 ) 5.3 × 10 3 e -4098/T m 6 /(mol 2 ‚s). The kinetic constant is essentially independent of the pH in the range 5 < pH < 8. When applying a NaCl concentration of 15 kg/m 3 , the kinetic constant increases ∼35% (T ) 328 K). The oxidation of Fe II (EDTA) to Fe III (EDTA) is not the sole oxygen-consuming reaction. The extent of side reactions, possibly related to EDTA ligand degradation, is a function of the pH and the oxygen concentration. A remarkable, unprecedented step change in the selectivity of the reaction was observed when the reaction was performed at a higher oxygen concentration.