Neutron imaging experiments were carried out to measure the water content of an operating proton exchange membrane fuel cell (PEMFC) under varying conditions of current density and temperature. It was found that the water content of the PEMFC is strongly coupled to the current density and temperature of the cell. These measurements indicate that changes in water content lag changes in current density by at least 100 s, both when the current density was increased and decreased. Less liquid water was measured in the cells when operating at 80 degrees C than at 40 degrees C. At 60 degrees C cell temperature, a peak in water content was observed around 650 mA/cm(2) and the water content was found to decrease with increasing current density. This is explained in the context of cell heating by performing a simple thermal analysis of an operating PEMFC so as to yield quantitative information on the waste heat and its effects on the liquid water contained in the cell. (c) 2006 The Electrochemical Society.