Neutron imaging experiments were carried out to measure the water content of an operating proton exchange membrane fuel cell ͑PEMFC͒ 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°C than at 40°C. At 60°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.Understanding liquid water content and its distribution within an operating proton exchange membrane fuel cell ͑PEMFC͒ is critical to designing high-performance systems and formulating rational models for simulating PEMFC behavior. The generation, transport, and removal of liquid water are key phenomena that occur in a PEMFC. Effective water transport through and removal from the membrane electrode assembly ͑MEA͒ is crucial to achieving high current density and maintaining PEMFC performance. In the design and optimization of PEMFCs, it is important to be able to quantify the water content in an operating cell in order to gain insight into the dominant phenomena or processes that influence liquid water transport and removal. This work is concerned with the measurement of liquid water in an operating PEMFC under various temperatures, relative humidities, and current densities. Neutron imaging, or radiography, is a useful tool for gaining qualitative and quantitative insight into liquid water content and distribution in near real-time ͑temporal resolution ϳ1 s͒.Both Tuber et al. 1 and Yang et al. 2 used optical methods for imaging water in PEMFCs under a range of operating conditing. In order to use optical techniques, a transparent fuel cell must be fabricated. Optical imaging is capable of high spatial and temporal resolutions for the elucidation of dynamic processes, but optical techniques suffer from fogging of the transparent window under humidified conditions and it is more difficult ͑though possible͒ to obtain quantitative information. Furthermore, optical investigations are limited to studying liquid water in the gas flow channels because that is the only visible water in the fuel cell; liquid water inside the gas diffusion layers ͑GDLs͒ cannot be imaged using optical techniques. Tuber et al. 1 were able to correlate the appearance of water in the cell with a drop in current density, although they did not quantify the liquid water in the cell. Yang et al. 2 focused on the appearance and dynamics of liquid water droplet formation and breakup in the gas flow channels. Their work ...