In this study, a generic electrical circuit is presented to characterise the frequency response of the Polymer Electrolyte Fuel Cell (PEFC), Cathode Catalyst Layer (CCL) at different current densities. The new electrical circuit is derived from fundamental electrochemical and diffusion theory. It consists of a transmission line in combination with distributed Warburg elements. The validation of this study is divided into a theoretical validation and an experimental validation. In the theoretical validation the impedance response of the CCL generated from three different circuits reported in the literature was compared with the simulated data from the generic electrical circuit. In the experimental validation, Electrochemical Impedance Spectroscopy (EIS) measurements were carried out in an H-2/air PEFC through a three-electrode configuration in the measurement system and were compared with the simulated data from the generic circuit. The results show that the generic circuit is able to accurately reproduce the measured data of the CCL at different current densities and is able to represent the electrochemical and diffusion mechanisms of the CCL in the frequency domain. It is possible to generate a deeper understanding of how and where the chemical energy that is released from the redox reaction is being dissipated and retained within the real physical system. (c) 2013 Elsevier B.V. All rights reserved.