Kinetics of exchange processes between the sodium ions from zeolite A and cadmium ions from solutions containing different contents of Na+ ions was determined by measuring changes in the concentrations of cadmium and sodium ions in both zeolite and the liquid phase during the exchange processes. The exchange kinetics were analyzed in accordance with the kinetic model derived on the basis of the second-order forward reaction between the cadmium ions from solution and the sodium ions from zeolite A and on the second-order backward reaction between sodium ions from solution and cadmium ions from zeolite A. The equilibrium cadmium uptake on zeolite A decreases with increasing concentration of sodium ions in the liquid phase. Agreement between the measured exchange kinetics and the exchange kinetics calculated by numerical solutions of the model equations shows that the exchange process takes place in accordance with the proposed model.