A three-dimensional numerical model is used to predict the local current density and membrane conductivity inside a 10 cm 2 polymer electrolyte membrane fuel cell with a serpentine flow path. The model includes the gas diffusion layer, and it accounts for the area hidden from direct contact with the flowing gases by the current collector contacts (i.e., the ribs). The predictions agree with experimental data for various operating cell temperatures at a fixed inlet humidity condition. Data on closure of the water balances are presented and are also shown to be consistent with the numerical predictions of water transport by electro-osmotic drag and back diffusion. The data and the model show conditions where insufficient water lowers the conductivity of the membrane and yields low currents at a fixed voltage. Finally, predictions of the distributions of current density, water flux per proton, and membrane conductivity are presented. (C) 2003 The Electrochemical Society.