The potentiometric response of the Na+ ion selective electrode based on NASICON (Na3Zr2Si2PO12) as well as the impedance of the NASICON/solution interface have been measured in NaCl. LiCl and KCI aqueous solutions. The behavior of the NASICON was successfully explained on the basis of the Eisenman model which involves an ion exchange process between the membrane and the solution. The apparent ion-exchange densities and potentiometric selectivity coefficients of the NASICON membrane have been determined for Na+, Li+ and K+ ions. The higher the apparent standard ion-exchange current density is, the better is the selectivity. However, no simple relationship can be established between these two parameters.