The effect of the solvent composition for water-ethylene glycol (EG) mixtures in the mole fraction range X(EG) = 0-1 on the diffusion coefficients and formal potentials of the ethylenediamine-N,N,N’,N’-tetraacetatovanadate(III/II) redox reaction has been investigated at a mercury electrode using convolution potential sweep voltammetry. It has been shown that the formal potential of the redox reaction, measured on the ferrocene/ferricinium scale, depends on the solvent composition. This dependence has been characterized by a two-step solvent exchange mechanism, which describes the solvation of the reactant and product. Distribution diagrams for the differently solvated species have been calculated as a function of the mole fraction of EG. Preferential solvation of the more negatively charged vanadium(II) complex relative to the vanadium(III) complex by water has been found. Acceptor numbers for the mixtures have been proposed.