In order to visualize the liquid water in a PEMFC, in-situ soft X-ray radiography technique has developed. Low energy X-ray in soft X-ray range makes it possible to visualize the liquid water in a PEMFC. The temporal and spatial resolution of the technique were improved and the real-time imaging was realized. Through-plane and in-plane visualization tests of an operating PEMFC were carried out. The results of through-plane visualization tests showed a strong separation of the liquid water between the under rib area and the under channel area, and non-uniform distribution of liquid water with the range of several dozens to hundreds of mu m was observed. In addition, the liquid water discharge behavior near under the rib area was observed by an in-plane visualization test. These results suggested the importance of a detailed understanding of the liquid water transport processes inside the PEMFC, and the soft X-ray radiography makes this investigation possible. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.