The objective of this work is to investigate phase-chage-induced water transport of polymer electrolyte fuel cell materials subjected to a temperature gradient. Contrary to thermo-osmotic flow in fuel cell membranes, a net flux of water was found to flow from the hot to the cold side of the full membrane electrode assembly. The key to this is the existence of some gas phase in the catalyst layer or other porous media. This mode of transport is a result of phase-chage-induced flow. The measured water transport through the membrane electrode assembly is the net effect of mass diffusion as well as thermo-osmosis in the membrane, which moves counter to the direction of phase-change-induced flow. Arrhenius functions that are dependent on material set, temperature gradient, and average temperature across the materials were developed that describe the net flux. In addition to direct quantification, phase-chage-induced flow was visualized and confirmed using high-resolution neutron radiography. (c) 2009 The Electrochemical Society. [DOI: 10.1149/1.3046136] All rights reserved.