A model has been developed for the prediction of multi-component ion exchange equilibria which uses equilibrium data from the constitutive binary systems to predict behaviour in multi-component systems. Non-idealities in the aqueous solution phase are considered by applying Pitzer's model for electrolyte behaviour while the non-ideal behaviour of the exchanger phase IS ccounted for by applying the model of Melis which assumes that the exchanger consists of two different types of exchanger sites, each having its own equilibrium constant for a binary system. The sites are present on the exchanger in equal amounts and are assumed to behave ideally. When applied to cation exchange in the binary, ternary and quaternary systems involving H+, Na+, K+, Ca2+ and Mg2+ ions, the model predicts well the equilibrium behaviour. The predictions for multi-component ion exchange equilibrium behaviour made by the proposed model are compared with those of the existing models proposed by Melis et al. and Mehablia et al.