A statistical thermodynamic approach proposed by Salmon for uni-bivalent and uni-tervalent cation exchange reactions has been applied successfully with the introduction of an empirical correction factor into the selectivity expression. The use of such an empirical factor is justified. From the equilibrium experiments, the thermodynamic equilibrium constant (lnKT), the additional energy term (ω) and the standard free energy of exchange have been evaluated. The enthalpy of exchange (ΔH0) has been obtained by calorimetric measurements. The influence of concentration and cross-linking of the exchanger on the heat of exchange have been studied and the experimental data reported. The influence of different coions on selectivity is shown. A remarkable and quite universal relationship between selectivity (ΔG0) and the water content of the resinates has been obtained for the uni-bivalent and uni-tervalent systems with four different cross-linkings; also the water content of the resinates in the potassium form falls on the point corresponding to a standard free energy of exchange of zero (ΔG0 = 0). This correlation is found to apply equally well with anion exchange reactions carried out previously in these laboratories. Such correlation lends support to the theoretical approach used in the present work. The additional energy term, w is dependent on the external electrolyte concentration and also on the cross-linking of the exchanger. With the data obtained for uni-bivalent cation exchange reactions, it was possible to predict bi-bivalent exchange reactions. The agreement found between the predicted and experimental values again supports the validity of the model employed.