The influence of polyaniline (PANI) layer thickness and applied potential on impedance characteristics was investigated. The impedance response of PANI layers, presented in the low frequency region of Nyquist spectra, indicates a decrease of real and imaginary impedance values as the thickness of the PANI layer increases. Presentations of EIS data in Bode diagrams show two potentially dependent separate processes, detected in the low and in the high frequency regions. The model used to analyze EIS data in terms of an electrical equivalent circuit resulted in PANI pseudocapacitance and PANI resistance values, which were used to calculate time-constants for each of the processes. The total PANI pseudocapacitance, calculated from PANI EIS parameters, was compared to the capacitive current plateau. The changes in EIS parameters were used to interpret processes taking place inside PANI and at the PANI I electrolyte interface. It was found that the PANI-free volume has a significant influence on the counter-ion flux inside PANI. The PANI-free volume was increased when more positive potential was applied. The effect of the PANI-free volume on capacitance and resistance characteristic, responsible for an impedance response in the low frequency region, was established. The impedance response of the PANI layer was changed upon prolonged ageing of the PANI layer in the supporting electrolyte. The ageing resulted in a decrease of the PANI free volume, which consequently resulted in an increase of ionic charge-transfer resistance responsible for the counter-ion flux. (C) 2007 Elsevier B.V. All rights reserved.