Transparent polymer electrolyte fuel cells (PEFCs) with parallel-and serpentine-channel flowfields have been developed to study liquid water formation and transport on the anode side. In situ observations reveal that liquid water in the anode channels results from condensation of water on cooler and more hydrophilic channel walls and that water available for condensation in the anode comes either from the cathode through membrane transport or from hydrogen consumption. No water droplets can be found on the anode gas diffusion layer (GDL) surface, in sharp contrast with the cathode side. Moreover, GDL wettability has much influence on water distribution in the anode for the cell operated at low current density (0.2A/ cm(2)). Using hydrophobic GDL at low current density, water is prone to condense on the channel surfaces rather than inside hydrophobic carbon paper GDL. The condensed water then accumulates and results in channel clogging in the anode. In contrast, by using untreated carbon paper as the anode GDL, it is found that channel clogging by liquid water is avoided under similar operating conditions. This finding implies that water distributions in the anode with hydrophilic and hydrophobic anode GDLs differ much from each other at low current density. Finally, experiments suggest that water condensation on the channel surfaces is a primary mechanism for liquid water formation and anode flooding and that elevating the anode plate temperature modestly is effective to avoid surface condensation and mitigate anode flooding. (c) 2007 The Electrochemical Society.