Considerable ongoing research exists in the area of fuel cells for power and distributed power generation. Of the various types of fuel cells, phosphoric acid fuel cell (PAFC) is a mature technology and is under limited production in various parts of the world. Electrochemical impedance spectroscopy (EIS) is a powerful tool which can be used for characterization of PAFC cathodes. In EIS, the electrode response is analyzed against an equivalent electrical circuit for diagnostics. A shortcoming of this approach is that multiple circuits can match the same response and correlating the physical electrode parameters with the circuit components may become quite difficult. To avoid this, a mathematical model of the PAFC cathode that is easy to solve is developed. The relative value and position of the maximum phase angle with respect to frequency is proposed as a diagnostic marker. A diagnostics table based on this marker is developed using the simulation of the mathematical model and the results are experimentally verified. (c) 2006 Published by Elsevier B.V.