A new analysis of scanning electrochemical microscopy (SECM) data is proposed in the case of a substrate covered by a film (porous material or electrolyte film,...) with diffusion-limited reactions. In a lot of experimental situations, the approach curve (measure of the current as function of the tip-substrate distance) is alike the one for a bare substrate with an irreversible kinetics reaction. In all these cases, this work demonstrates a very simple relationship between the apparent irreversible kinetics parameter extracted from the approach curve and film characteristics, giving the ratio between the permeability and the thickness of the film. The validity domain of the new data analysis is carefully determined. It is then applied to experimental data on Nafion films taken from literature. (c) 2008 Elsevier B.V. All rights reserved.