Voltammetric studies of polypyrrole (PPy) films doped with Cl-, Br-, NO3-, and ClO4- anions under precisely controlled conditions show that the redox activity of the polymer film is determined by the nature of the anions used during the electrodeposition and is almost independent of the anions used in the electrochemical studies. The mobilities of these anions are rather close but still have a well-defined order: ClO4- < Br- < Cl- < NO3-. Quantum chemical calculations also reveal differences in the strength of interaction of the anions with pyrrole oligomer dications. (C) 2004 Elsevier Ltd. All rights reserved.