The cathode flooding has a significant impact on DMFC performance. Liquid water reduces diffusion of oxygen through the cathode GDL. Oxygen transport problems on the cathode side redistribute the rate of methanol consumption and eventually lead to transport limitations on the anode side. Our model shows that there is a critical level of flooding of the cathode backing layer below which the cell performance suffers greatly, eventually running the risk of cell reversal. This critical point is reached when the oxygen diffusivity in the flooded domain drops by a factor of five with respect to the normal non-flooded level. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.