Effective management of liquid water produced in the cathodic reaction of a polymer electrolyte membrane (PEM) fuel cell is essential to achieve high cell efficiency. Few experimental methods are available for in situ measurements of water transport within an operating cell. Neutron radiography is a useful tool to visualize water within a cell constructed of many common materials, including metals. The application of neutron radiography to measurements of water content within the flow field channels of an operating 50 cm(2) PEM fuel cell is described. Details of the experimental apparatus, image processing procedure and quantitative analysis are provided. It is demonstrated that water tends to accumulate in the 180 degrees bends of the serpentine anode and cathode flow fields used in this study. Moreover, the effects of both the current density and cathode stoichiometric ratio on the quantity of accumulated water are discussed. (c) 2006 Elsevier Ltd. All rights reserved.