This paper discusses a novel approach for description of ion-exchange processes accompanied by substantial sorption of coions by using the new generalized notations such as “the superequivalent ion exchange” and.“the superequivalent sorption”. The thermodynamic description of superequivalent exchange as a general case of ion-exchange process is based on the non-osmotic theory of ion exchange. The general methods for calculation of ion-exchange constants and activities of ions in the resin phase are considered and discussed. A new method for calculation of these quantities based on the necessaiy and sufficient thermodynamic description of an ion-exchange system is proposed. The method gives new possibilities to verify the thermodynamic compatibility of the experimental results. The proposed theory has been applied to treat experimental results obtained by studying the ion-exchange equilibrium in CaCl2- NaCl - H2O system on Dowex 50×8, KU-2×8 and Dowex 50×2 resin in a wide range of compositions. A new method for the graphical presentation of ion-exchange equilibrium data in the form of the isotherm cross-sections is proposed and discussed. The comparison of the description of ion-exchange equilibrium in the system under study in terms of the absolute and relative (“Gibbs excesses”) sorption values of all system components (including counterions, coions and water) has been carried out. The results of calculation of ion-exchange constants, activities and activity coefficients of resinates accomplished by different methods are presented and discussed. A possibility to model ion-exchange isotheims and activity coefficients of resinates is demonstrated and discussed.