Oxidation of ferrocene at a poly(3-octylthiophene) (POT) film electrode has been investigated by using cyclic voltammetry, rotating disc electrode voltammetry and electrochemical impedance spectroscopy. The POT film electrode wats prepared by galvanostatic electropolymerization of 3-octylthiophene on a rotating Pt disc electrode. In the medium used i.e. 0.1 M LiBF4 + propylene carbonate, the formal redox potentials of ferrocene and POT are 0.4 and 1.1 V vs. Ag/AgCl respectively. This makes it possible to study the oxidation of ferrocene at both undoped (semiconducting) POT and at doped (electronically conducting) POT. The voltammetric data show that the oxidation of ferrocene is kinetically controlled at undoped or lightly doped POT while ferrocene is oxidized at a diffusion limited rate at highly doped POT. Impedance data on the POT film electrode in solutions containing different concentrations of supporting electrolyte and ferrocene indicate that the ferrocene oxidation occurs in parallel with the doping process, resulting in a competition between ferrocene and doping ions for the available ’sites’ at the polymer/solution interface. The kinetics of electron transfer at the polymer/solution interface are discussed based on the Butler-Volmer formulation of electrode kinetics.