Water management has been an important issue in the operation of hydrogen PEM fuel cells (Murahashi et al., 2006; Lu et al., 2007). If the water content in the membrane is too low, membrane proton conductivity and hence cell performance can be adversely ažected. On the other hand, when the water produced from the cell reaction at the cathode cannot be removed e¥ciently, water œlls up the pores in the electrode, blocking access of oxygen to the cathode catalyst. šis is referred to as ›ooding of the electrode. Understanding how water is transferred in a fuel cell can provide insights in proper water management to avoid ›ooding and dry-out conditions.