In this paper, electrochemical impedance spectroscopy ( EIS! is used to resolve various sources of polarization loss in a pure hydrogen-fueled polymer electrolyte fuel cell (PEFC). EIS data are fitted to a fuel cell model in which the catalyst layer physics are accurately represented by a transmission line model. Extracted parameters include cell ohmic resistance, catalyst layer electrolyte resistance, and double-layer capacitance. The results showed that the catalyst layer electrolyte resistance for a state-of-the- art electrode (47 wt % Pt on Vulcan XC-72 carbon, 0.8 Nafion (1100EW)-to-carbon weight ratio, 13 mu m thick) at 80 degrees C and fully humidified conditions was approximately 100 m Omega-cm(2); this translates to a dc voltage loss of about 33 mV at a current density of 1 A/cm(2). Similar results were obtained for two experimental methods, one using H-2 (anode) and O-2 (cathode gas feed) and another with H-2 and N-2 supplies, and for two cell active areas, 5 and 50 cm(2). The measured catalyst layer electrolyte resistance increased with decreasing ionomer concentration in the electrode, as expected. We also observed that the real impedance measured at 1 kHz, often interpreted as the ohmic resistance in the cell, can include contributions from the electrolyte in the catalyst layer. (c) 2005 The Electrochemical Society.