A model has been developed for the prediction of multicomponent ion exchange equilibria which uses equilibrium data from binary systems to predict behaviour in multicomponent systems. Non-idealities in the solution phase are considered by applying Pitzer’s model for electrolyte behaviour while the resin phase activity coefficients are calculated by applying Wilson’s model. In addition the equilibrium constants are obtained from the Gaines and Thomas approach and are independent of the resin phase activity coefficients. The model also considers the non-availability of some ions for exchange due to the formation of ion pairs in the solution phase. When applied to cation exchange in the binary and ternary systems involving H+, Na+, K+ and Ca2+ ions, the model predicts well the equilibrium behaviour. The equilibrium constant and the two binary interaction parameters are found to be independent of the conditions of the solution phase including concentration and the particular non-exchanging anionic species present in the solution.