Electrochemical impedance spectra obtained for poly(o-phenylenediamine) films in contact with solutions of different compositions were analyzed on the basis of selected models. The Kramers-Kronig transformation has been used for validation of the impedance data. Complex non-linear least squares fitting and computer simulation have been used to derive reliable quantities characterising the electrochemical behaviour of these systems. It has been shown that the charge transport diffusion coefficient strongly decreases with increasing potentials. The potential dependence of low-frequency capacitance attests to strong interactions within the film that depends upon the nature of anions. The participation of protons in the redox reactions has also been demonstrated.