A model fully coupling the flow, species transport, and electrochemical kinetics in polymer electrolyte fuel cells is presented to explore operation undergoing very large density and velocity variations. Comparisons are also made to a previous constant-flow model, which neglects the mass source/sink from the continuity equation and assumes constant gas density. Numerical results reveal large density (>50%) and velocity (>80%) variations occurring in the anode at anode stoichiometry of 1.2. In addition, the hydrogen concentration remained as high as the inlet owing to deceleration of the anode gas flow. Finally, the constant-flow model is accurate within 14% under common operating conditions, i.e., for anode stoichiometry ranging from 1.2 to 2.0. (C) 2005 The Electrochemical Society. All rights reserved.