For the electric potential and electrode selectivity of membranes based on solvent mixtures, expressions were obtained which allow for the transport of cations in the form of solvates with these solvents. The model was tested for polyvinyl chloride membranes containing mixtures of dioctyl phthalate and trioctyl phosphate in different concentration ratios. There is mutual agreement between data for the stoichiometry of cation solvation by trioctyl phosphate obtained by analysis of electrode properties and those obtained by analysis of extraction properties of the membranes.