The redox reaction Fe-III + e <----> Fe-II is studied at a Nafion(R)-filmed Pt electrode using both steady-state and EIS methods. It is shown that the fitting of the EIS experimental data can be greatly improved by using the new concept of a 'theoretical-experimental hybrid model', i.e., by introducing experimental data obtained at a bare electrode in the theoretical model for a filmed electrode. It is also shown that a combined steady-state and EIS study cannot lead to a separate determination of the three key parameters of the system: the diffusion coefficient in the polymer film (D-f), the thickness of the film (L), and the partition coefficient between the solution and the polymer (gamma), only the two parameters gammaD(f)/L and L-2/D-f being obtained from experimental data by a curve fitting procedure. Finally, an exhaustive theoretical EIS study in quiescent solution is also proposed. (C) 2003 Elsevier B.V. All rights reserved.