Synchrotron X-ray radiography was employed to visualize the distribution of liquid water throughout an operating polymer electrolyte membrane fuel cell (PEMFC). The behavior of liquid water in the fuel cells assembled with and without micro-porous layers (MPLs) was compared. Through-plane images under various operating conditions combined with PEMFC performance revealed that, under wet conditions, the MPL was associated with an increased number of breakthrough locations, resulting in higher operating potentials. With these results, it is suggested that the MPL prevents liquid water from agglomerating into larger clusters at the catalyst layer/MPL interface, thereby reducing the number of blocked pathways for oxygen transport. (C) 2012 Elsevier B.V. All rights reserved.