Measurement results obtained from single-cell experiments give insight of electrochemical processes and allow for their optimization. However, the operator of large fuel cell stacks is confronted by a different set of problems that do not arise in such small scale experiments. Typically in a fuel cell stack the reactants and the cooling medium are fed in parallel to the cells. This can lead to an uneven flow distribution in the flow channels and an uneven cell voltage distribution across the stack. Therefore, a cleverly devised control and monitoring system is required to ensure that no unbalanced strain is put on the stack. This paper investigates some aspects critical to the operation of large fuel cell stacks in automotive applications such as control issues in the supply system, stack failures, and the appropriate countermeasures as well as some procedures to increase the voltage stability. (C) 2003 Elsevier B.V. All rights reserved.