The membrane potential arising through charged porous membranes separating electrolyte mixtures at identical hydrostatic pressures but different concentrations was investigated theoretically by means of the steric, electric, and dielectric exclusion (SEDE) model. Transport phenomena were described through the Nernst-Planck formalism, while ion partitioning at the membrane/solution interfaces was accounted for by means of modified Donnan equations including steric and dielectric effects. The high concentration limit of the membrane potential depends on the mixture composition and the pore size as well. A specific feature of membrane potential ill multi-ionic systems is the dependence at high concentration on the effective dielectric constant of the Solution confined inside pores. Indeed, the effective dielectric constant inside pores does not affect the high concentration limit of the membrane potential in the case of single salt solutions. The low concentration limit of the membrane potential is independent of the Mixture composition, the effective dielectric constant inside pores, and the pore radius, but it is ruled by counterions with the highest charge number. The membrane potential measured at high salt concentration with single salt solutions and electrolyte mixtures could be used to determine the pore size and the effective dielectric constant inside pores, respectively. This may constitute an alternative way for membrane characterization with the advantage of avoiding the need for additional rejection rate measurements.