Based on conservation equations, a physical model for impedance of the cathode catalyst layer (CCL) in a fuel cell is developed and analyzed. In all the cases, the CCL impedance spectrum is an ideal or distorted semicircle linked to the straight 45 degrees-degree line in the high-frequency domain. Spectrum distortion (elongation along the real axis) manifests the regime with doubling of apparent Tafel slope, resulting from the proton- or oxygen-transport limitations in the system. In the case of ideal oxygen transport, the spectra corresponding to different currents do not intersect, while the oxygen transport loss manifests itself as an intersection of spectra. This feature provide a simple means for understanding the regime of CCL operation. (C) 2012 Elsevier B.V. All rights reserved.