Relationships between corrected selectivity coefficient (K-C) and fraction of the exchanged cadmium ions in zeolite A (f(Cd,z)) were determined from data obtained by measuring the concentrations of sodium and cadmium ions in both the solid and the liquid phase at equilibrium of the exchange processes between cadmium ions from solution and sodium ions from zeolite A at different total ion concentrations and different temperatures. Thermodynamic equilibrium constants (K-a) calculated from the corresponding Kielland＇s plots (K-C vs f(Cd,z) plots) were used for the calculation of the appropriate values of standard free energy (Delta G degrees) standard enthalpy (Delta H degrees), and standard entropy (Delta S degrees). The influence of the degree of exchange on the activity coefficients of sodium and cadmium ions in zeolite is also considered.