The outer micro-porous layer (MPL) between the gas diffusion layer and channel of the bipolar plate was studied for both sides of the electrodes in DMFC, with particular attention to the effects of the hydrophobicity of the MPL on mass transport as well as cell performance. Water-transport behavior from the electrodes to the channel was observed through the transparent window of the single cell with membrane-electrode assemblies (MEAs) including three combinations of outer MPLs. The crossover amount of methanol as well as water through the membrane was measured, and the mass balance, based on the measured flux, was established to understand the mass transport in MEAs. The design of outer MPLs is discussed for the best cell performance. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.