A model has been developed to obtain the thermodynamic parameters, i.e., the equilibrium constant and solid-phase activity coefficients, that describe binary ion exchange reactions. The model is based on the thermodynamic framework for adsorption exchange reactions developed by Gaines and Thomas (1953. Journal of Chemical Physics, 21, 714-718). The Wilson activity coefficient model is used for the adsorbed phase and the stoichiometry of the exchange reaction is taken into account. The solid solution model parameters are regressed with a new data reduction technique, based on the use of the Vanselow selectivity coefficient calculated at various points on the isotherm equilibrium surface. Additionally, a consistency test is developed for analyzing the selectivity coefficient plots. For the estimation of the selectivity coefficient, the Bromley-Zemaitis model is used for calculating the activity coefficients in the aqueous phase. Also, the speciation of the aqueous systems is taken into account. The equilibrium constants are calculated using their relationship with the selectivity coefficients and the activity coefficients in the solid phase. In addition to accurate representation of experimental data, the data reduction of binary pairs from three ternary systems has been shown to satisfy the triangular rule consistency test. Uptake graphs of the investigated systems have been accurately predicted.