A simple model is presented to describe the permeation of methanol from the anode to the cathode in direct methanol fuel cell (DMFC). Measured permeation rates of water and methanol through Nafion(R) 117 under varied pressure differentials across the membrane are used to determine key parameters in the model. This model is able to explain the effect of oxygen pressure at the cathode and methanol concentration at the anode on the measured cell voltage-current response of the DMFC.