The first application of Electrohydrodynamical (EHD) impedance [1,2] to the study of the transport properties of the charge carriers in modified electrodes is presented in this note. The specific interest of this technique, which involves a modulation of the rotation speed of a disc electrode, as compared with ac impedance, lies in that the flux of charges injected in the electroactive polymer layer is modulated at a constant potential. It has been shown recently that EHD impedance is a suitable technique for elucidating mass transport of a redox substrate by diffusion through an inert (non-conductive) film [3]. Commonly adopted models for modified electrodes, see e.g. ref. 4, are based on (i) a redox reaction of the mediator couple confined in the polymer layer: P+e-PQ (ii) a redox reaction between the substrate in solution and the mediator: Q + Ak’\P + products (for a reduction) Most generally, four processes may concur to limit the overall reaction rate: (1) substrate (convective) diffusion in solution; (2) substrate molecular diffusion in the layer; (3) charge transfer between Q and A and (4) “diffusion” of electrons in the layer. The location of the mediation reaction at the polymer/ solution interface or in the layer is a result of the relative rates of the above four processes.