In this study neutron radiography is applied to investigate the performance of a polymer electrolyte membrane (PEM) fuel cell based on the effect of liquid water accumulation in the cell. Dynamic performance tests have been carried out on a PEM fuel cell with a specially designed serpentine flow channel under various operating conditions and simultaneous measurements of accumulated liquid water with neutron imaging. Liquid water tends to accumulate in the gas diffusion layer (GDL) adjacent to the flow channel area while the liquid water formed in the GDL next to the channel land area seems to be effectively removed by the cross leakage flow through the porous GDL between the adjacent flow channels. The amount of liquid water accumulation in the cell is dependant on the cell operating temperature, the pressure drop in the flow channel and the current density under the present test conditions of fixed stoichiometry. it is shown that the cell performance is strongly affected by the presence and accumulation of liquid water, especially at high current densities. This phenomenon results in performance hysteresis for load variations. The rate of liquid water production is also mathematically modeled to analyze the effect of the cell operating temperature and pressure drop on the liquid water formation in a cell. The model result shows good agreement with experimental measurements. The history of liquid water accumulation is also analyzed. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.