Eight different ion exchange equilibrium models are tested to determine which gives the most accurate predictions of equilibrium behavior for multicomponent ion exchange. The eight models differ only in the way in which they account for solution phase phenomena. Nonidealities in the exchanger phase are accounted for by applying the Wilson model. The solution phase models considered include a model that assumes ideal solution behavior and others that apply the extended Debye-Huckel equation, the Meissner and Kusik electrolyte solution theory, and the Pitzer electrolyte solution theory. Each of these methods for accounting for solution phase nonidealities are applied twice, once assuming incomplete ion dissociation and once assuming that ion dissociation was complete. The models are tested using an extensive set of equilibrium data for the 10 binary, 10 ternary, 5 quaternary, and 1 quinternary systems involving,exchange of the five H+, Na+, K+, Mg2+, and Ca2+ cations on a commercial resin. The results show that for nearly all of the systems studied the greatest single improvement in the accuracy of the model predictions is achieved when incomplete dissociation of the ions is considered even if neither the extended Debye-Huckel equation nor the models of Meissner and Kusik or Pitzer are applied to the solution phase. The application of Pitzer's theory yields significantly superior results to those of Meissner and Kusik and the other models.