The oxidation-reduction of the Ferri/Ferrocyanide couple in solution Onto modified glassy carbon Rotating Disk Electrodes (RDE) covered by Os(II) bipyridile poly-vinylpyridile (OsBPP) polymer was Studied at room temperature. Steady state polarization curves were carried out as a function of the rotation speed, the polymer thickness and the concentration of redox centers within the polymer. This system has the characteristic that the formal redox potentials of both the external redox Couple (E-0'(Fe(CN)(6)(3-/4-)) = + 0.225 V vs. SCE) and the mediator polymer (E-0'(OsBPP) = 0.260 V vs. SCE) lie very close. It is demonstrated that diffusion of the Ferri/Ferrocyanide inside the polymer can be ruled out. Since the processes of charge transfer at the metal/polymer and the mediating reaction are fast, the experimental results can be interpreted in terms of a kinetics in which the charge transport in the polymer or the diffusion in the solution may be the rate determining step, according to the experimental conditions. A simple model is considered that allows interpreting the experimental results quantitatively. Application of this model allows the determination of the diffusion coefficient of the electrons within the film, D-c approximate to 10(-10) Cm-2 s(-1). (C) 2008 Elsevier Ltd. All rights reserved.