PANI films of thicknesses from 0.06 to 7 mu m were investigated by the potentiostatic pulse method and EIS. The films behaved very similar in respect to both methods over the whole range of thicknesses. The oxidized film can be described as a battery electrode with a specific capacity as high as 130Fcm(-3). The series resistance of the oxidized film is much smaller than the electrolyte resistance. The reduced film has a high series resistance of about 4k Omega cm. For the reduced him the capacity of the interface metal-polymer is detected in the impedance spectra. Both impedance spectra and potentiostatic current transients indicate that the oxidation of PANI is an inhomogeneous process. We conclude from our measurements, that the transition follows a sequence of nucleation and growth of conducting zones, perpendicular to the substrate electrode. Partially oxidized polymer films can be frozen-in. The impedance spectra of these intermediate states are interpreted on the basis of a geometrical model.