In this paper, temperature dependent performance of a Nafion 112-based proton exchange membrane (PEM) fuel cell was investigated at different temperatures, 30 psig back pressure, and 100% relative humidity (RH). High cell performance of ca. 0.637 V at 1.0 A/cm(2) was obtained at 120 degrees C. Cell voltage decreased when the temperature increased within the range of 80-20 degrees C. An in situ ac impedance spectroscopy method under load was developed to diagnose the performance reduction. A semi-empirical treatment was initiated to obtain expressions for extinguishing the individual performance drops caused by reaction kinetics (charge-transfer resistance), membrane resistance, and mass-transfer limitation, respectively. (c) 2006 The Electrochemical Society.