The design and operation of a differential polymer electrolyte membrane (PEM) fuel cell is described. The fuel cell design is based on coupled stirred tank reactors (STRs) coupled through a membrane; the gas phase in each reactor compartment is well mixed. The characteristic times for reactant flow, gas phase diffusion, and reaction were chosen so that the gas compositions at both the anode and cathode are uniform. The STR PEM fuel cell is one-dimensional; the only spatial gradients are transverse to the membrane. The cell is used to examine start-up, and dynamic responses to changes in load, temperature, and reactant flow rates. Multiple time scales in the system's response are found to correspond to water absorption by the membrane, water transport through the membrane, and stress-related mechanical changes of the membrane. (C) 2004 American Institute of Chemical Engineers.