Homoionic K-clinoptilolite was evaluated as both sink for NH4+ and as a source of nutrient such as K+ through a thermodynamic model. In this model, the isotherm for the exchange reaction, K-z + (NH4+)(s) reversible arrow = (NH4)(z) + K-s(+) for different size fraction of clinoptilolite (20-50 and 50-75 mu m) was obtained at various charge fractions of NH4+ + K+ solution but at total variable concentration of 0.01, 0.1 and 1.0 M. From the thermodynamic data obtained, the mass action quotient (K-m) values, corrected with activity coefficient (K-c) were calculated. These data were plotted as log K-c vs. the equivalent fraction of (NH4)(z) in zeolite to derive the thermodynamic equilibrium constants (K-a). The standard Gibbs free energy Delta G(0) was calculated. From the obtained K-a and Delta G(0) values, it was found that the exchange reaction was favoured at lower concentration (0.01 M) and for small particle size fraction (20-50 mu m) of clinoptilolite. The maximum selectivity coefficient (alpha) values of clinoptilolite (20-50 and 50-75 mu m) to NH4+ from aqueous solution were found to be 2.4 and 1.9, respectively at lower concentration of 0.01 M.The effect of anions background on the selectivity of natural clinoptilolite to K+ and NH4+ was studied by calculating the Gapon selectivity coefficient (K-G). The activities of NH4Cl and KCl used in the exchange solutions were calculated by using Ion Speciation Model (MINTEQ A(2)/PRODEFA(2)). The use of CsCl together with the solution mixture of KCl and NH4Cl helps the exchange of NH4+ to K+ due to the role of Cs+ in displacing K+ ions. The importance of this study stems from the fact that it throws light on thermodynamics exchange process of NH4+ to K+ and vice versa on the surface of clinoptilolite and consequently between environment containing NH4+ and soil amendment containing K+.