Neutron radiographic imaging is combined with locally resolved current density measurements to study the effects of local water content on the performance of the corresponding electrochemical active area in an operating PEM fuel cell. Liquid water agglomerates are detected, quantified and correlated with the activity of the respective area. At low currents, depletion of the reactant gas leads to a decreasing performance along the anodic flowfield channel. At high currents, an optimum humidification is reached in the central part of the fuel cell; close to the inlets respectively outlets, flooding and drying can be observed concurrently and cause a non-uniform current density distribution across the reactive area. The fast response of the local performance on water droplets migrating in the gas channel is tracked by short-term imaging taking place on a timescale of several seconds. (c) 2007 Elsevier B.V. All rights reserved.