The variations of the selectivity coefficient K A B between Na + –H + , Na + –K + , and Na + –Cu 2+ systems and the separation factor α A B between Na + –Cu 2+ and K + –Cu 2+ systems in cation-exchange membranes as functions of loading and particle size of resin have been measured. The exchange affinities of all the membranes increase as H + <Na + <K + <Cu 2+ . K A B is almost constant, independent of particle size and loading of resin in the membrane. α Na Cu and α K Cu increase with decrease in particle size but remain almost constant with loading of resin in the membranes. The order of selectivity is K Na H <K Na K ≤K Na Cu . The values of (α A B Q 0 −1 ) other (α A B Q 0 −1 ) −1 M(300)C60 for the Na + –Cu 2+ system are approximately 1.060 and for K + –Cu 2+ are approximately 0.870. Q 0 represents the exchange capacity of a dry membrane per unit volume (in milliequivalents per cubic centimeter).