Polarization curves, i.e., dc performance, and impedance spectral signatures of polybenzimidazole (PBI)-phosphoric acid (H3PO4) membrane fuel cells are obtained in the temperature range of 160-180 degrees C, in an effort to investigate the effect of temperature, anode humidification, various cathode stoichs, and use of oxygen versus air. Thus, in situ electrochemical impedance spectroscopy (EIS) was used to obtain various resistances, ohmic as well as charge-transfer resistances, under these conditions. The results obtained show that PBI-H3PO4 gel membrane fuel cells exhibit very good performance in the temperature range of 160-180 degrees C with an ohmic resistance similar to Nation. Mass transfer limitations were determined by comparing performance polarization curves with air and oxygen along with EIS. Further EIS was also used to obtain signatures during fuel starvation, and electrical shorting across the cell. (c) 2006 Elsevier B.V. All rights reserved.