Electrochemical behavior of the alpha-[SiMo3-xVxW9O40]((4+x)-) and alpha-[PMo3-xVxW9O40]((3+x)-) anions with x = 1, 2, or 3 were studied. Electrochemical reduction of each compounds was consistent with its MoN ratio, reduction of vanadium and molybdenum atoms occurring in the +0.6 to -0.6 V potenfial range. The one-electron-reduced species were prepared by electrolysis and then characterized by ESR spectroscopy. The g and A values for V4+ ions appeared to depend on the nature of the surrounding atoms (Mo-VI, W-VI, and V-V). In solution at 330 K, the ESR spectrum of the protonated alpha-H[(SiMoVVW9O40)-V-IV](6-) anion displayed 29 superhyperfine lines which were related to the partial localization of the electron on one vanadium nucleus. The ESR spectra at room temperature for the divanadium-substituted anions showed a strong anisotropy of the A tensor which would be related to the electron transfer along a preferential axis. An isolated V4+ signal was not observed, even at 12 K, indicating that the electron is never firmly trapped on one single vanadium atom.