The electrochemical doping process of platinum phthalocyanine (PtPc) microcrystalline films in acetonitrile electrolyte has been investigated using electrochemical impedance spectroscopy (EIS). The system shows the impedance behaviour expected for a conductive polymer-that is, the appearance of a separate Randles circuit, a Warburg section and purely capacitive behaviour at low frequencies, An equivalent circuit is developed which provides a good fit to experimental impedance data over a wide frequency range of 1 MHz-0.05 Hz. The kinetic parameters of the electrochemical doping process depend strongly upon the doping potential. Analysis of the conductivity of the PtPc film suggests that a percolation effect is responsible for the first-scan discrepancy. At low doping levels, the rate of the first electrochemical step is slow and determined by the conductivity of the microcrystalline film, Once the film becomes conductive, the electrochemical reaction is accelerated abruptly giving rise to a sharp peak. Further increases in doping potential trigger another slow oxidation process. The potential dependence of the diffusion-migration capacitance suggests strong interactions between charge carriers within the microcrystalline film.