A new technique is described for electrochemical impedance spectroscopic (EIS) investigations in the plane of a polymer layer coating an electrode without crossing the interface polymer-electrolyte. For this purpose a twin-working electrode configuration was constructed, that consists of two practically identical electrodes with a gap of a few micrometers width between them. This gap was bridged by a polymer layer of polybithiophene. Its behaviour was studied during electrochemical deposition on the twin electrode and during transition from insulating to conducting state upon oxidation. It is shown that for lower frequencies the impedance spectra obtained depend predominantly on the polymer conductivity and are not determined by the macroscopic external polymer/electrolyte interface. Contributions of capacitive ac currents that cross microscopic internal polymer-electrolyte interfaces in the deep pores of the polymer become considerable only in the higher frequency range.