It was found that operating a proton-exchange membrane fuel cell at elevated temperature and pressure could largely activate it. When the fuel cell was tested again at ambient conditions, its performance was much higher than that before the activation procedure was performed. Without the activation, the fuel cell could probably never achieve the enhanced performance if only a traditional break-in procedure was performed. The activation procedure normally did not need to exceed 2 h, and it was applicable to all the catalysts, electrodes, catalyst-coated membranes (CCMs), and membrane-electrode assemblies studied. (C) 2003 Elsevier Science B.V. All rights reserved.