Synchrotron X-ray radiography is employed to visualize the temporal evolution of water inside the gas diffusion layer (GDL) of an operating (in situ) polymer electrolyte fuel cell (PEFC). A single-cell PEFC test kit is specially designed for the convenient capture of X-ray images. X-ray images of water in the PEFC components, such as the polymer membrane, GDL, and end plate, are captured consecutively. The synchrotron X-ray radiography of high-spatial and high-temporal resolution is suitable for observing the transport of a liquid layer and for visualizing water distribution inside the PEFC. As a result, the spatial distribution of water in the PEFC components is clearly and quantitatively visualized. The temporal evolution of water in the anode GDL due to back diffusion effect is clearly observed by adopting the image normalization method. The water-saturation characteristics at the cathode GDL, including saturation time and speed, are quite different from those at the anode GDL. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.