Most analyses of fuel cell stacks consider either transport or electrochemical phenomena, but neglect the coupling between the two. An original mathematical model was developed to solve the transport equations coupled with heterogeneous electrochemical reactions for large stacks with internal/external manifolds. The code was used to determine the effect of utilization on electrochemical performance and temperature distribution. The results are presented in terms of fluid flow, pressure, temperature, voltage, and current density distributions. The discussion highlights the importance of manifold design on performance and thermal management.