A mathematical model for the cathode of a direct methanol fuel cell (DMFC) is developed to investigate two-phase transport in the catalyst layer (CL) and to elucidate the mechanism of cathode mixed potential due to oxidation of crossover methanol. A coupled model of two-phase species transport and multistep electrochemical kinetics, including simultaneous oxygen reduction, methanol oxidation, and gas phase chemical reaction, is presented. The model predictions agree favorably with experiments of cathode mixed potential, and the predicted profiles of water saturation, oxygen concentration, and overpotential along the CL thickness further reveal the profound interplay between multiple reactions and the transport of oxygen and water. It is shown that in the presence of methanol crossover, the DMFC cathode is operated at higher overpotential and water saturation, with larger oxygen transport loss than that in the H-2/air counterpart. The model results also indicate that reducing the cathode CL thickness can facilitate both liquid water removal and oxygen transport through the CL, leading to improved cathode performance. (c) 2007 The Electrochemical Society.