A new technique is presented to characterize and quantify the two-phase flow in the anode and cathode flow field channels using simultaneous anode and cathode visualization combined with image processing. In situ visualization experiments were performed at 35 degrees C with stoichiometric ratios (an/ca) of 1.5/2.5, 1.5/5, 3/8 to elucidate two-phase flow dynamics at lower temperature/low power conditions, when excess liquid water in the cell can be especially prevalent. Video processing algorithms were developed to automatically detect and quantify dynamic and static liquid water present in the flow field channels, as well as discern the distribution of water among different two-phase flow structures. The water coverage ratio was introduced as a parameter to capture the time-averaged flow field water content information through recorded high speed video sequences. The automated processing allows for efficient and robust spatial and temporal averaging of steady state channel water over very large visualization data sets acquired through high speed imaging. The developed algorithm calculates the water coverage ratio using the liquid water in the channels which is contacting the GDL surface, and selectively removes the superficial condensation on the visualization window from the coverage area. The water coverage ratio and distribution metrics techniques were demonstrated by comparing the performance of Freudenberg and Toray gas diffusion layers (GDLs) from a water management perspective, including direct anode to cathode comparisons of simultaneous water coverage data for each GDL sample. The anode water coverage ratio was found to exceed the cathode for both GDL samples at most operating conditions tested in this work. The Freudenberg GDL consistently demonstrated a higher water coverage ratio in the flow field gas channels than the Toray GDL, while the Toray GDL indicated a propensity for greater water retention within the membrane electrode assembly (MEA) based on performance, high frequency resistance (HFR), and water coverage metrics. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.