CO2 gas produced in a DMFC during methanol oxidation reaction is typically removed out of the cell through the anode channel while liquid fuel mixture is supplied from the anode channel to the anode porous media. This induces a two-phase flow, which makes it difficult to understand mass transport phenomena in a DMFC. This paper discusses the effect of CO2 on two-phase flow and mass transport phenomena in a DMFC with a multi-dimensional model. The role of CO2 in controlling water and methanol transport in DMFCs is theoretically investigated. Multi-D DMFC model explains that amount of CO2 in the anode controls not only water transport by capillary diffusion, but also methanol transport by both molecular diffusion and capillary diffusion. Two DMFCs (conventional cell and CO2 breathing cell) are fabricated and tested to confirm the theoretical result acquired from the simulation. (C) 2014 Elsevier Ltd. All rights reserved.