An electrode flooding monitoring device designed for PEM fuel cells with interdigitated flow distributors was proposed and tested. The pressure drop between the inlet and outlet channels can be used as a diagnostic signal to monitor the liquid water content in the porous electrodes, because of the strong dependence of the gas permeability of the porous electrodes on liquid water content. It can be used with no modification to existing fuel-cell assembly to give real-time flooding information in the electrode backing layers during operation. The device has been employed to investigate the correlation between the fuel-cell performance and the liquid water saturation level in the backing layers, the effects of various operating parameters, and the dynamics and hysteresis behavior of liquid water in the backing layers. The results provide, for the first time, direct evidence to show that inadequate water removal causes liquid water build up in the cathode, which in excessive amounts can severely reduce the performance of a PEM fuel cell. It was observed that more than 30 min. were needed for a PEM fuel cell to reach a new steady state when subjected to current-density changes, and this was attributed to the slow liquid water transport process. The results confirmed that increasing air flow rate or cell temperature increased the liquid water removal rate from the backing layers. The hysteresis behavior of fuel-cell performance was related to the water imbibition and drainage cycles in the electrode backing layers and was attributed to the difference in the water removal rate by capillary force and the difference in membrane conductivity.