The performance of a commercial 300W(el) self-humidifying polymer electrolyte fuel cell (PEFC) stack was investigated by studying polarization curves under different operating conditions (temperature, stoichiometry). It could be demonstrated that internal humidification has a major impact on stack performance. Especially when the stack was operated in the low power range at elevated temperatures, dehydration was a serious problem. By applying extreme operating conditions, complete MEA drying-out as well as flooding could be observed. Knowledge of the underlying physical and chemical interrelationships is fundamental for the optimum application and control of polymer electrolyte fuel cell stacks in energy systems. (C) 2004 Elsevier B.V. All rights reserved.