The potentiometric response of electrochemically coated polypyrrole layers was studied both in hydrochloric acid and in alkali metal chloride solutions. The H+ ions were found to exert an influence, different from that of alkali metal cations, resulting in a positive slope of the open-circuit potential vs. log[H+] dependence. This was found also for concentrated KCl solutions, where the Donnan potential is close to zero. Voltammetric and coulometric experiments demonstrated that this behaviour cannot be explained only by the acid-base properties of polypyrrole. Additionally, redox reactions between the polymer film and solution components, whose equilibrium constants (and thus final open-circuit potential) may be pH dependent, should be taken into consideration. On the basis of model calculations, the influence of the concentration of redox species on the potential of a polymer electrode is discussed.