A systematic method for the steady-state modeling of mass transport phenomena and electrochemical behavior focusing on the cathode side of a direct methanol fuel cell (DMFC) with an open-cathode design is presented and demonstrated using an experimental system. The architecture features an internal water-recovery technology that promotes back-convection of water from the cathode to the anode through a polymer electrolyte permeable membrane that includes laser-drilled holes. Eight adjustable modeling parameters are estimated from experimental data from a prototype system, and a model validation study shows that predictions of water venting rates and of cell voltage have errors better than 9%9% when extrapolating to conditions not used for parameter estimation. Illustrations of the use of the model include an analysis of performance at various flow rates of feed air, operating temperatures and currents, and a prediction of the effect of alternative material designs for selected cell layers. The approach serves as a paradigm for the development of general models for open-cathode active-feed DMFC systems.