Abstract: The Oxidation-reduction behaviour of H₃PMo₁₂O₄₀, M₃PMo₁₂O₄₀ (M = NH₄, Na, K), H₄PMo₁₁VO₄₀ and M_n,H_4-nPMo₁₁VO₄₀(M = Na, La, K, Eu; n=1～4 for Na and La, n=4 for K, n= 2 for Eu) with a Keggin structure was studied by an electrochemical method in the mixed solvents of water-dioxane containing supporting electrolyte of 0. 5 mol/L H₂SO₄. The molybdenum in the acids and in their salts can gain or lose eight electrons, the vanadium in them can gain or lose one electron. The order and reversibility of oxidation- reduction for molybdenum and vanadium were also analyzed.

Key words: Heteropolyacid, Redpx, Molybdophosphoric heteropolyacid, Molybdovanadophosphoric heteropolyacid