The usual expression of the finite Warburg element used for analyzing electrochemical impedance spectroscopy spectra is obtained assuming that the reaction occurs at the electrode/membrane interface and that the oxygen concentration at the gas channel/gas diffusion layer interface is constant. A simple improvement of this expression consists in the oxygen concentration depletion along the gas channel. This pseudo-two-dimensional approach should be more appropriate for the investigation of mass transfer limitations in membrane-electrode assemblies of polymer electrolyte membrane fuel cells (PEMFC). Starting from experimental data from the literature, numerical simulations show that conclusions about the mass transfer limiting layer and its main characteristics can be significantly modified, which can contribute to a better understanding of oxygen transport in fuel cells. The results also put forward the existence of a critical value of the air stoichiometry below, which, close to the air channel exit, no oxygen can access to the active layer. However, the diffusion impedance model does not take into account time-dependent current variation effects on the gas concentration, although experimental proofs by Schneider et al. [J. Electrochem. Soc., 154, B383 (2007); 154, B770 (2007)] brought that they influence significantly the shape and the size of the measured impedance spectra. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3481560] All rights reserved.