Poly(3-methoxythiophene) (PMOT) films of different thicknesses (0.25–0.75 μm), electrodeposited on Pt from an aqueous anionic micellar medium, were characterized by electrochemical impedance spectroscopy (EIS) measurements in aqueous LiClO 4 solutions. Both the oxidized and the neutral (chemically reduced) forms were investigated. Impedance spectra were obtained at potentials ranging from 0 to 0.8 V/SCE. The EIS data were fitted using an equivalent-electrical circuit in order to characterize the PMOT electrochemical properties. A progressive increase of the different components of the circuit with the film thickness was observed. With the applied potential, we obtained utmost capacitances and least resistances at the open-circuit potential. We showed also that the neutral form of PMOT behaves like an insulator. The study of the influence of the electrolyte concentration demonstrated that the diffusion of ClO 4 − anions is the rate-determining step of the doping process. The presence of a pore resistance suggests that the film should be highly porous and contain an excess of electrolyte, which facilitates anion-diffusion. The EIS results elect PMOT as a promising active material for batteries.