The corrosion behaviour of mild steel covered by the electrodeposited polypyrrole films was investigated by electrochemical impedance spectroscopy. Electrodeposition of polypyrrole was performed from aqueous solution of pyrrole and oxalic acid at a constant current density of 1 mA cm(-2) and yields strongly adherent and smooth polymer layers. The impedance spectra were treated through the nonlinear least squares (NLS) fitting to estimate the parameters of the proposed equivalent circuit, based on physical model for the corrosion of mild steel protected by polypyrrole films in acid sulfate solutions.