A proton exchange membrane (PEM) fuel cell with a flow field that can be switched between the serpentine and the interdigitated flow modes was used to measure the liquid water saturation level in the gas diffusion layer (GDL) of the cathode and the effect of the liquid water saturation level on the fuel cell performance. Using correlations between the liquid water saturation level and gas relative permeability obtained by neutron imaging, the liquid water saturation level in the GDL under serpentine flow mode was determined by the gas pressure drop across the GDL right after the flow field was switched from the serpentine mode to interdigitated mode. The results showed that the saturation levels in the cathode GDL during the interdigitated mode was much lower than that during the serpentine mode leading to better oxygen gas access to the cathode catalyst layer and consequently better fuel cell performance, especially at high current densities and low oxygen stoichiometric flow rate. In most cases, the fuel cell became unstable when the average liquid water saturation level exceeded 20%. (C) 2011 Elsevier B.V. All rights reserved.