A one-dimensional transmission-line model for porous electrodes is adopted to analyze AC impedance spectra obtained from polymer-electrolyte-membrane (PEM) fuel cells. The model considers a Tafel-type non-linear expression for the charge transfer resistance; the proton transport resistance and the double-layer capacitance along the thickness of the cathode catalyst layer are also included. By comparing the model results with the experimentally obtained AC impedance data of a PEM fuel cell, it is demonstrated that the model can successfully predict the shape and the magnitude of impedance spectra and also be used to characterize polarization performance of the cell as an in-situ diagnostic method. As an outcome of the analysis, the contribution of the proton transport resistance of the cathode catalyst layer is quantified, and its contribution to the shape of the impedance spectra is analyzed. In addition, the data analysis methodology that can quantify the lump-summed magnitude of oxygen diffusion rate through the electrolyte layer in the cathode and the apparent exchange current density of oxygen reduction reaction is proposed. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.053302jes] All rights reserved.