Relationships between the corrected selectivity coefficient, K-C(Me) (Me = Zn, Pb), and fraction of the exchanged Me2+ ions in zeolite A, f(Me,Z), were determined from the corresponding exchange isotherms. The exchange isotherms were obtained by measuring the equilibrium concentrations of sodium and Me2+ ions in both the solid and the liquid phase at constant total-ion concentrations and different temperatures in the range from 20 degreesC to 60 degreesC. Thermodynamic equilibrium constants, K-a(Me), calculated from the corresponding Kielland's plots (lnK(C)(Me) vs. f(Me,Z) plots) were used for the calculation of the appropriate values of standard free energy, DeltaG degrees (Me), standard enthalpy, DeltaH degrees (Me), and standard entropy, DeltaS degrees (Me).