Electrochemical impedance spectroscopy (EIS) was used to study the capacitance and ion transport properties of fuel cell catalyst layers. It was found that limiting capacitance correlates with active area. The capacitance per gram of catalyst was calculated and is proposed as a measure of catalyst utilization. Results obtained with catalyst layers immobilized on glassy carbon electrodes agree very well with results obtained with gas diffusion electrodes. EIS was also used to study ion conductivity and active area in fuel cell electrodes that contain the electroactive probe Os(bpy)(3)(2+). Together, these results validate the hypothesis that the non-ideal behavior of fuel cell electrodes is due to variations of conductivity across the layer, rather than variations in capacitance. (c) 2004 Elsevier Ltd. All rights reserved.