AbstractWhen the ratio of electrical transport numbers (ratio of fluxes) of two ionic species in a membrane has a value different from the ratio of concentrations in free solution, the membrane may be utilized to effect a partial separation of the two species. The degree of separation may be characterized by a ‘separation factor’, a, which has been evaluated theoretically for a simple case. The effect of current density on a is also considered. The theory is utilized to determine the optimum conditions for the separation of amino‐acids from sodium chloride in a three‐compartment cell employing ion‐exchange membranes. Results are in general agreement wi