The management of water is a challenging issue to achieving high power density, long-term operation, and increased robustness in PEMFCs. Development of in-situ diagnostic techniques to understand the dynamic behavior of liquid water is needed. In this study, visualization of liquid water across the membrane electrolyte assembly (MEA) of operating PEMFCs was performed by using high-resolution soft X-ray radiography, with which was possible to distinguish the catalyst layer (CL), polymer electrolyte membrane, and gas diffusion layer (GDL) of the MEA. Dynamic liquid water transport behavior in the cathode CL and GDL was observed at high spatial and temporal resolution (1 frame/s). At low current density, generation of liquid water was recognized under the rib, while at high current density, liquid water was observed both under the rib and the channel. Temporal behaviors of liquid water accumulation in GDL under the rib and discharge process of liquid water to under the channel were also visualized. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.