The conversion process between one- and two-electron waves for [PMo12O40](3-) was investigated in a binary mixture of acetonitrile and solvent S (S = NAT-dimethylformamide (DMF), dimethylsulfoxide (DMSO), H2O, propylene carbonate (PC), or 1,4-dioxane) containing 0.10 M (M = mol dm (-3)) LiClO4. A reversible two-electron wave was obtained in a binary mixture of acetonitrile and 1,4-dioxane. On the other hand, the first two-electron wave was converted into two one-electron waves in a mixture of acetonitrile and DMF, DMSO, H2O, or PC. The Li-7-NMR studies demonstrated preferential solvation. of Li+ by one of the solvents with stronger Lewis basicity. The electrode process was explained on the basis of the relative permittivity (8,) of the mixed solvent and the relative donicity of acetonitrile and solvent S. It turned out that the number of transferred electrons at the electrode could be controlled by a proper choice of binary solvent mixtures.