The behavior of fuel cells (FCs) at steady state and during transients is an important factor both for control tuning and for performance assessment. In the technical literature, few papers deal systematically with FC characterization. In this paper, the performance characterization of a proton exchange membrane (PEM) FC is carried out using an experimental analysis. The experimental activity has been conducted in a test station, properly designed and able to test PEM FC stacks in the range of 500-2000 W. The laboratory test facility is equipped with a National Instruments CompactDAQ real-time data acquisition and control system running a LabVIEW software. The bench commands two mass flow controllers, regulating both fuel flow and air flow which are commanded via two Recommended Standard 232 ports. The temperature of the FC is regulated via a fan operated by a brushless motor drive. An electronic load is connected to the FC terminals. The main operating parameters, such as the air stoichiometric ratio and fuel composition, have been varied and measured, and their influence on the PEM FC behavior has been investigated under both steady-state and transient conditions.