Ac impedance technique was used to study the electrochemical behaviour of polyelectrolyte-sodium interface, in which the polyelectrolyte has a single-Na+-ion conductivity of 3.5 X 10(-6) S/cm at 25 degrees C. Impedance diagram of platinum electrode system consists of a depressed semicircle and a spur, whereas that of sodium electrode system has three incomplete semicircles. This difference suggests there is an ion-conducting passivation layer on the polyelectrolyte-sodium interface, which is characterised by the semicircle of the impedance diagram of sodium electrode system at intermediate frequency. Time dependence of the impedance spectra reveals that the formation of passivation layer is due to the reaction between polyelectrolyte and metal sodium, related to the storage time after the assembly of cells. The passivation layer was found to have a comparable resistance with that of the bulk polyelectrolyte, and hence have little effect on electrochemical process of the cells. Using impedance data, dielectric property of the polyelectrolyte media was discussed, and impedance spectra were explained as well.