The selectivity coefficients between chlorides and nitrates are studied experimentally as functions of the degree of ionization of the membrane, of ionic strength, and of the ionic equivalent fraction of chloride in solution. Through the use of Harned's rule, a simple expression of selectivity between chlorides and nitrates is found to apply with constant interaction coefficients. Swelling and co-ion molalities in Donnan equilibria with mixed external solution of the same cation and different anions are predicted from equilibrium properties with pure electrolytes and from the “net-charge” densities.