A comprehensive description of proton exchange membrane fuel cell (PEMFC) performance includes the transport phenomena, phase change and electrochemical reaction inside the several components, which possess disparate characteristics and together form a complex three-dimensional geometry. Much of the modelling work in this area has, therefore, relied on the techniques of computational fluid dynamics(CFD). The comprehensive three-dimensional (3D) approach can, however, be prohibitively time consuming. Consequently. it is not the ideal basis fora rapid screening tool that operates under a wide range of design options and operating conditions. Mathematical models and solution procedures using simplified models with reduced dimensions have been proposed to address this issue. Such approaches are computationally efficient, but no systematic study has been conducted to qualitatively or quantitatively assess the impact of the neglected dimensionality on the accuracy of the resulting model. In this paper, we compare results from a hierarchy of reduced-dimensional models to the results from a comprehensive 3D CFD model for a single. straight-channel unit cell. The quality of the simulation results from reduced-dimensional models, including the cell voltage and the distributions of current density and relative humidity, are assessed. We demonstrate that the 2 + 1 D approach, which includes mass transport in the 2D cross-section of the channel and membrane electrode assembly and integrates along the flow channel, is optimal in terms of both efficiency and accuracy. It provides a sound basis for a simulation tool that can be used in the early stages of a unit-cell design cycle. (C) 2009 Elsevier B.V. All rights reserved.