A steady and unsteady water and thermal model was developed to consider the effects of local pressure on the cell performance, pressure drop, open-circuit voltage variation with stack temperature, and water-vapor effects on membrane conductivity. These considerations made the model physically more reasonable as well as more suitable for various operating conditions. Additionally, this model combined the along-flow-channel model and catalyst layer model, which represent a significant improvement to proton exchange membrane fuel cell (PEMFC) modeling. The model could predict the distributions of a series of important parameters along the flow channel and in the catalyst layer. Furthermore, the transient performance of the fuel cell can be simulated with this model. The modeling results agreed reasonably with the available experimental results from the literature. The results show that the humidification of both anode and cathode is very important for the performance of PEMFC which could also be improved by increasing the flow inlet temperatures within a reasonable range. Pressure loss is one of the important parameters that affect total system efficiency and optimization. This model could be used as part of a PEMFC stack or entire system modeling. (c) 2006 The Electrochemical Society.