The two-step one-electron reduction processes of Keggin-type heteropolymolybdate and heteropolytungstate anions were investigated in several solvents using cyclic voltammetry. The slope of a plot of standard redox potentials E(o) against the ionic charge Delta E(o) was evaluated. It was found by theoretical considerations that Delta E(o) consists of a constant term independent of the solvent and another term comprising the charge-dependent component of the solvation energy. The difference between Delta E(o) for water and for 1,2-dichloroethane (1,2-DCE) was in good agreement with the value estimated from the transfer energies across the 1,2-DCEwater interface which were measured by ion transfer voltammetry. The values of Delta E(o) in various solvents cannot be explained by the Born-type electrostatic solvation energy, Suggesting that the short-range interaction is important for the interaction of a Keggin anion with solvent molecules.