In the present work, the detailed thermal and water management in the membrane of proton exchange membrane fuel cells (PEMFC) is investigated numerically. The coupling effects of mass diffusion and temperature gradient on the water distribution in the membrane are taken into account with consideration of the temperature-dependent diffusivity. Thermal and water transport equations with various boundary conditions are solved by the control volume finite difference method. Predictions show that under the conditions of fixed water concentration at the cathode side, the effect of cathode temperature, T-c, on the water concentration is significant. Increases in T-c may lead to an increase in membrane dehydration. At the water-flux condition on the cathode side, the influence of the operating temperature on the water distribution in the membrane shows a similar trend. The effects of the anode temperature, T-a, on the water management in the membrane are also examined. It is found that T-a has considerable impact on the water content in the membrane. In addition, high current density may cause non-uniformity of the temperature distribution in the membrane. (C) 2003 Elsevier B.V. All rights reserved.