Ultra thin (10-20 nm) and pin-hole free overoxidized polypyrrole films were directly electrosynthezised under potentiostatic conditions in the presence of weak-acid anions, specifically monohydrogenophosphate. Various overoxidized polypyrrole membranes were fabricated in these conditions at different potentials in the range of 0.70 to 0.90 V/SCE. Their non-conductive property and blocking character toward anions were verified by cyclic voltammetry and electrochemical impedance spectroscopy in the presence of the ferricyanide/ferrocyanide redox couple. The results show that these films are definitively ultra thin, pin-hole free and non-conductive membrane. For comparison purposes, classical polypyrrole films were also prepared in the presence of perchlorate ions, overoxidized in KOH solution, and finally electrochemically characterized. (C) 2013 The Electrochemical Society. All rights reserved.