The formation of Fe3O4 and α-FeO(OH) during the course of the air oxidation of Fe[OH]2 suspensions has been studied at pH 11.0 and 65 °C. When the sulphate concentration is low (0.03 mol dm–3), the oxidation proceeds in three stages: (1) formation of iron(III) oxides and slower formation of Fe3O4; (2) rapid formation of Fe3O4; and (3) linear formation of Fe3O4. When the sulphate concentration is high (0.60 mol dm–3), α-FeO(OH) is formed accompanied by a slow formation of amorphus γ-FeO(OH). The oxidation rates observed in stationary state are due to the oxidation of FeII adsorbed on the surface of the solid phase.