A model is presented of a polymer electrolyte fuel cell including slow transient effects of liquid water accumulation and evaporation in gas diffusion electrodes (GDEs) and gas channels. The model is reduced dimensionally, coupling a one-dimensional (1D) model of gas and coolant channel flow to 1D models of transport through the membrane electrode assembly (MEA) and bipolar plates. An asymptotic reduction of the two-phase flow to a sharp interface model is used, in which phase change occurs at a front that evolves in time. The asymptotic reduction is based on an immobile water fraction in the GDE and a large capillary pressure. The water content in the membrane and channels is also tracked in time. Gas and thermal transport are taken to be at quasi-steady state on the time scale of liquid accumulation. The model is fit to Ballard Mk9 cells and validated against experimental measurements of both steady-state and transient MEA water content distributions along the length of the channel. Predictions of slow cyclovoltammograms are presented based on the model. (c) 2008 The Electrochemical Society.