A careful investigation of the effect of solution ionic strength on the ion-exchange behavior of zeolites that was conducted by R. Harjula and co-workers demonstrated that mass-action-based formulations failed to anticipate the equilibria encountered once the ionic strength of the solution in contact with the zeolite was reduced to less than or equal to 1.0 x 10(-3) M. The results of this research have been analyzed employing Gibbs-Donnan-based concepts in an attempt to understand better the nature of this phenomenon. Complications introduced by zeolite hydrolysis reactions became amenable to analysis through application of the Donnan model. Whereas this led to a more knowledgeable assessment of the hydrolysis phenomenon than the earlier empirical treatment by Harijula and co-workers, failure to anticipate ion-exchange behavior at low ionic strengths still persisted. Reasonable rationalization of this failure, unavailable to the mass-action-based formulation used earlier by Harijula and co-workers, was, however, provided by the Gibbs-Donnan model.