This paper reports on an experimental approach to measuring the water crossover coefficient distribution for the first time. A straight-channel PEFC of 6 cm(2) segmented into 10 pieces along the reactant flow direction has been developed. The instrumented cell, combining the functions of current collection and gas sampling with one pin for each segment, is capable of measuring current and species distributions simultaneously from a single experiment. The two distribution data are subsequently combined in a material balance analysis to yield the net water transport coefficient distribution through the membrane. For fully humidified anode and partially humidified cathode, the net water transport coefficient is found to range from 0.47 to 0.025, and the electro-osmotic drag dominates water transport through the membrane. For partially humidified anode and cathode, the net water transport coefficient lies between 0.19 and -0.24, with the negative value indicating dominant back diffusion. (c) 2006 Elsevier B.V. All rights reserved.