A mathematical model of slow transient behavior in an autohumidified stirred tank reactor (STR) polymer electrolyte membrane (PEM) fuel cell is developed. The key feature of the model is the positive feedback between current, water production, and membrane resistance which leads to two stable "ignited" states, corresponding to either a uniform current distribution or a partially ignited cell with localized current production. The switching between the two regimes is accompanied by hysteresis and transient behavior on the order of 2-4h in a small 1 cm(2) cell. We compare the numerical results to experimental data gathered by [Benziger et al. 2005. Chemical Engineering Science 60 (6), 1743-1759] and show that the lateral diffusion of water within the ionomer membrane is a possible mechanism behind the hysteresis and slow transient behavior they observed. (c) 2005 Elsevier Ltd. All rights reserved.