Many applications of ion-exchange membranes involve polyionic transfers between multicomponent electrolyte solutions. Polyionic membrane cell potentials are usually interpreted in terms of individual ionic activities, i.e. nonthermodynamic, conventionally established quantities. A new, complete scheme is here described for evaluating emf’s of polyionic membrane cells in terms of thermodynamic mean ionic activities in mixed electrolytes together with the relevant ion and solvent transference numbers. Applications of the proposed scheme are described for (a) the determination of transference numbers of ions and solvent across membrane, and (b) the determination of selectivities of membranes to be used in ion-selective electrodes.