A non intrusive method based on small angle neutron scattering (SANS) has been developed to determine the water concentration profile through the thickness of Nafion 117 membrane during fuel cell operation. This technique was used to study the effect of gas flow configuration, co- or counter-flow, on water repartition within the fuel cell both within and outside the membrane. As it has been reported previously in the literature the counter-flow configuration gives better performance than co-flow but more surprisingly we evidence a significant difference in performance between symmetric configurations either in co- or counter-flow. Indeed, for a given current density, cell voltage is higher when the cathode inlet is at the bottom of the cell. We demonstrate that the gravity retains liquid water within the cell which leads to a better membrane hydration. Moreover, we have been able to correlate the average water content within the membrane with the performance and especially with the voltage drop resulting from the membrane resistance. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.