Electrochemical impedance spectroscopy (EIS) at low polarization was used successfully to assess the relative performance of ion-exchange membrane pairs for high-current electrodialysis of sodium nitrate solution. A comparison was based on the resistances calculated from high-frequency impedance measurements at cell potentials not larger than 500 mV. Anionic membranes AMV, ACM, 3475 and AAV were paired with cationic membranes CMS, CMV and 3470 to identify the most efficient combination. Configurations involving the AAV anion-exchange membrane clearly showed higher resistance than the other membrane pairs, and their impedance plots presented a distorted semicircle at frequencies higher than 1 Hz. Fitting of an equivalent circuit attributed this semicircle to diffusional phenomena. Raman spectroscopy of an AAV membrane after exposure to the working solutions revealed a strong interaction between nitrate ions and the pyridinium groups of the membrane, which suggests trapping of the nitrate ion by the pyridinium group or sharing of protons between the two, thus retarding nitrate transport. (C) 2009 Elsevier B.V. All rights reserved.