A fundamental experimental and numerical study of the water transport across a perfluorosulfonic acid membrane under a temperature gradient is presented. The water transport phenomenon was experimentally investigated through water flux measurement and neutron radiography. The experimental observations found that water is transported in the direction from the high temperature side to the low temperature side, when both sides of the membrane are sufficiently humidified, and suggest that the transport mechanism is concentration gradient driven. The neutron radiography measurements detected the presence of water content gradient across the membrane and higher water content is seen at a larger thermal gradient. A numerical model was developed to investigate the experimental results. Water transport predictions agreed qualitatively but more accurate material and transport property characterizations are needed for further improvement. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3530794] All rights reserved.