and ac measurements were carried out to study the ion conductivity and permselectivity of the conducting polymer polypyrrole in dependence of the oxidation state. Free.-standing membranes were prepared by electropolymerization on a gold sputtered microporous polycarbonate foil which was mounted in a two-compartment measurement cell. The oxidation state of the ppy membrane was controlled potentiostatically by electron charge transfer in the asymmetrical configuration (gold layer/ membrane/electrolyte), and the ion conductivity and permselectivity were studied separately in the symmetrical configuration (electrolyte/membrane/electrolyte) by combining constant current permeation experiments with IC and AAS solution analysis. It was found that polypyrrole exhibits an appreciable ion conductivity and anion permselectivity in the oxidized state. A membrane with distinct cation permselectivity in the reduced state can be prepared, when anionic groups are fixed in the polymer either by incorporation of immobile counterions (dodecylsulfate) or by copolymerization of a modified pyrrole monomer carrying a sulfonate group (N-sulfopropyl-pyrrole). Depending on the relative concentrations of the positive charges (ppy') and the fixed negative charges (sulfonate groups) the copolymer membrane combines both anion and cation permselectivity. Since the concentration of ppy' can be adjusted electrochemically by oxidation or reduction of the polymer backbone, such a membrane can be switched dynamically between both ion conducting states.