A theoretical model describing the dependence of the open-circuit potential on electrolyte concentration for bilayers of conducting polymers is presented. One component of the bilayer is an anion-exchanger, while the second one is a cation-exchanging matrix. This model proposes diagnostic criteria concerning the properties of such bilayers : (i) the presence of redox reactions between component polymers, (ii) the absence of such reactions due to hindrances in ion transfer across the outer layer, necessary to maintain the electroneutrality of the polymer, (iii) the presence of defects in the outer coating. This model was applied to analyze the experimental results in the systems where the cation exchanging matrix was polypyrrole with poly(4-styrenesulfonate) ions or with indigo carmine, while poly(N-methylpyrrole) with perchlorate doping ions served as an anion exchanger. Potentiometric measurements were performed in solutions of KCl, MgCl2 and BaCl2. Although the potentiometric results in BaCl2 solution exhibited more pronounced anionic responses than in KCI solutions, it was found that during conditioning, prior to the potentiometric measurements, both univalent (K+) and divalent (Ba2+) could penetrate the outer PMPy layer.