A mathematical model is developed that examines the effect of having a constrained membrane in a fuel cell. During operation, a polymer-electrolyte fuel-cell membrane is prone to swell but is unable to do so by system design. This builds up a stress that affects transport in, and the properties of the membrane. This report is the first to incorporate such effects into a macrohomogeneous model. Both the physical and mathematical effects are described. Results include the magnitude of constraint, the change in water content of the membrane attributed to constraint, and one-dimensional simulations showing the effect of constraint on the water balance of the fuel cell. The paper demonstrates the need for considering membrane constraint both in modeling and experimental studies of fuel cells. (C) 2004 American Institute of Chemical Engineers.