Anode overpotentials in a commercial fuel cell have been measured using dynamic hydrogen reference electrodes (DHE) at the top and bottom of the cell. During operation of the cell with dry H-2 and O-2, which causes dehydration of the top of the anode, the top DHE consistently yielded higher anode overpotentials than the bottom DHE. Poisoning of the top part of the anode with CO greatly enhanced this disparity. Together with the observed time dependences, these results clearly indicate that overpotentials measured by using reference electrodes outside the active area of a fuel cell are local values. (c) 2006 The Electrochemical Society.