Various approaches to the interpretation of ion-exchange equilibria have been examined. In these approaches, units of concentration, standard states, reference states, and reference functions employed in the two phases either differ or are the same. In the first instance, useful composition dependent corrections for nonideality are necessarily empirical and do not provide a molecular level understanding of arrangements and interactions between the exchanging ions. The second path, accessible through the employment of Gibbs-Donnan-based logic, provides real insight, on the other hand.