Impedance measurements were taken of films of polythiophene which had been formed electrochemically on a platinum electrode. The polythiophene was measured in various states of oxidation, the doping anion being PF6-. The impedance measurements were of two types : one type involved the arrangement metal/polymer/electrolyte and the other was of the form metal/polymer/metal, with as much solvent removed as possible. From the latter the polythiophene in reduced form was found to have a conductivity of approximately 10(-10) OMEGA-1 cm-1, and in the oxidized form approximately 10(-4) OMEGA-1 cm-1, with the transport number for the electronic charge being at least 0.95. No evidence was found for anion motion in this case and the diffusion coefficient for the anion must be less than 10(-11) cm2 s-1. These measurements can be reconciled with the metal/polymer/electrolyte measurements only if it is assumed that in the latter case channels of electrolyte penetrate through the polymer almost to the metal causing the measured bulk conductivity of the polymer always to be above 10(-6) OMEGA-1 cm-1. In this case the anion appears to be mobile with an apparent diffusion coefficient of approximately 10