Electrochemical impedance spectroscopy (EIS) under controlled hydrodynamic conditions was used to determine the active zone of p-doped poly(3,4-ethylenedioxythiophene) (PEDOT) modified electrodes towards ferrocene in acetonitrile solution. For a ferrocene probe, the heterogeneous electron transfer occurred at the p-doped PEDOT film \ solution interface. EIS measurements were carried out below the limiting current. The faradaic impedance at low frequency was used to analyze the effect of the interface heterogeneity in term of an anomalous diffusion process. An anomalous diffusion exponent gamma of 1.11 was obtained. The value of the anomalous diffusion exponent indicated a super-diffusive regime of the mass transport. In acetonitrile solution, the heavy oxidized p-doped PEDOT film might be viewed as amorphous organic metals which induced an anomalous diffusion process in its vinicity for diffusion-convection limited reactions. In the absence of specific interaction between the interface and the electrochemical probe, the active zone of the PEDOT modified electrode was mainly controlled by the morphology and the doping level of PEDOT rather than the rate of the heterogeneous electron transfer. (C) 2003 Elsevier Science B.V. All rights reserved.