The swelling of a sulfonic ion exchange resin (ES 28; 8 % divinylbenzene) was measured for hydrogen, alkaline, and alkaline-earth counterions in water, or in equilibrium with solutions of the corresponding chloride, and also for mixed forms. Compared with the (1M) isotherms, the monomonovalent biionic mixed forms show linear variations of swelling with composition and verify the Rice and Harris model. For monobivalent biionic forms, the equal swelling-equal affinity relationship is only verified at high solution concentrations.