In-situ observation of liquid water transport in the microporous layer (MPL) and gas diffusion layer (GDL) of an operating proton exchange membrane fuel cell (PEMFC) was performed using high-resolution soft X-ray radiography. The amount of liquid water accumulated in the MPL and GDL increases with increasing in current density. The amount of liquid water in the MPL is low near the catalyst layer and higher toward the GDL side. The liquid water is transported to the GDL mainly through the cracks in the MPL The dynamic liquid water transport from the cracks in the MPL to the interconnected pores in the GDL was observed. (C) 2012 Elsevier B.V. All rights reserved.