This work constitutes detailed EIS (Electrochemical Impedance Spectroscopy) measurements on a PBI-based based HT-PEM unit cell. By means of EIS the fuel cell is characterized in several modes of operation by varying the Current density, temperature and the stoichiometry of the reactant gases. Using Equivalent Circuit (EC) modeling key parameters. such as the membrane resistance, charge transfer resistance and gas transfer resistance are identified, however the physical interpretation of the parameters derived from EC's are doubtful as discussed in this paper. The EC model proposed, which is a modified Randles circuit, provides a reasonably good fit at all the conditions tested. The measurements reveal that the cell temperature is all important parameter, which influences the cell performance significantly, especially the charge transfer resistance proved to be very temperature dependent. The transport of oxygen to the Oxygen Reduction Reaction (ORR) likewise has a substantial effect on the impedance spectra, results showed that the gas transfer resistance has all exponential-like dependency oil the air stoichiometry. Based oil the present results and results found in recent publications it is Still not clear what exactly causes the distinctive low frequency loop occurring at oxygen starvation. Contrary to the oxygen transport, file transport of hydrogen to the Hydrogen Oxidation Reaction (HOR), in the stoichiometry range investigated in this study, shows no measurable change in the impedance data. Generally. this work is expected to provide a basis for future development of impedance-based fuel cell diagnostic systems for HT-PEM fuel cell. (C) 2009 Elsevier B.V. All rights reserved.