The potentiometric pH response of electropolymerized polypyrrole modified glassy carbon electrodes as a function of polymer film thickness was examined. Electrodes coated with thicker polypyrrole films showed mixed potentiometric responses toward both cationic and anionic species, while thinner polypyrrole films (<2 mu m) showed good linear response to pH changes for the pH range between 2 and 12. The possible mechanism for such behavior was discussed.