Poly(3-octylthiophene) (POT) is a polymer which can be made electrically conductive by oxidation (p-doping). The presence of the octyl side chains makes this polymer melt-processable and soluble in common organic solvents. Films of POT were deposited on a Pt disc by electrochemical polymerization of the monomer and by solution-casting of chemically synthesized POT. The electrochemical properties of the Pt/POT electrodes were then studied in lithium tetrafluoroborate-propylene carbonate solution for various thicknesses of the polymer film. Impedance/admittance measurements using the Fourier transform faradaic admittance measurements technique were performed at d.c. potentials corresponding to neutral POT and to different reversible oxidation levels of POT, and the impedance data were fitted to an equivalent electrical circuit. It was thus possible to follow how the parameters related to charge transfer, diffusion and double layer changed with the oxidation level of POT.