A three-cell polymer electrolyte membrane fuel cell stack equipped with perfluorinated ionomer membranes is tested for 1000 h in severe operating conditions. The fuel cell (FC) is fed by dry hydrogen/low humidified air and it is simultaneously operated at overnominal temperature. During the long term experiment, some polarization curves and electrochemical impedance spectra are recorded at regular time-spaced intervals for various stoichiometry rates. The objective is to characterize the degradation of FC performances with the aim to better understand the FC aging mechanisms. Some response surfaces are plotted to display and analyze the stack voltage variations as a function of aging time, static load current, and gas utilization rates. Different points of the FC impedance spectra are also considered. The evolutions of their coordinates in the Nyquist plot can be monitored versus aging time and for different gas utilization rates.