The oxidation of americium in HNO3, H2SO4 and HClO4 solutions by a mixture of potassium persulfate with silver salt in the presence of potassium phosphotungstate has been investigated. The influence of acid and its concentration, of (NH4)2S2O3, K10P2W17O61 and silver salt on Am(III) oxidation rate, yield and stability of Am(IV) and Am(VI), has been studied. The complexation of Am(III), Am(IV) and Am(VI) with phosphotungstate ions has been investigated. It has been established that Am(III) and Am(IV) form ML2 complexes and their apparent stability constants have been estimated. The oxidation mechanism is discussed. A method for preparing of Am(IV) in 0.1–6M HNO3, O.1–3M H2SO4, 0.1–1M HClO4 solutions is proposed. The oxidation of Am(III) to Am(IV) by KBrO3 and K2Cr2O7 in HNO3, H2SO4, HClO4 solutions in the presence of K10P2W17O61 has been investigated.