Mg2+-selective polymeric membranes were studied by means of impedance analysis with regard to the bulk membrane resistance, the charge transfer resistance and the membrane selectivity. The dependence of the bulk membrane resistance on the presence of lipophilic anionic sites is clearly shown by impedance spectra. The mechanisms of berate catalysis and how they actively contribute to the membrane＇s interfacial charge-transfer process is elucidated. The membrane selectivity was investigated by impedance analysis. This is an alternative method to potentiometric measurements (not orthogonal), and can, unlike calorimetry or transport studies, be used to test ion-selective membranes under real conditions. (C) 2000 Elsevier Science Ltd. All rights reserved.