The objective of this work is to investigate water transport in polymer electrolyte fuel cell membranes subjected to a temperature gradient. Direct thermo-osmotic experiments on different polymer membrane types (non-reinforced Nafion (R), and reinforced Gore-Select (R) and Flemion (R) membranes) were performed using an in-house fuel cell fixture with a parallel flow field and pure membrane sheet without catalyst layers. Thermo-osmotic flow was observed in all membranes, and the water flow direction in the membrane was determined to always flow from the cold to hot side, as anticipated for a small pore hydrophilic porous medium. The water flux was found to be proportional to temperature gradient, and increase with average membrane temperature. The dependency of the thermo-osmotic diffusivity on average temperature showed predictable Arrhenius-type behavior. True interfacial temperatures of the membrane were estimated using a two-dimensional thermal model, and empirical relations for the thermo-osmotic diffusivity for the membrane types tested were developed. These can be of use to design engineers concerned about achieving optimal water balance during steady and transient operation. (c) 2008 Elsevier B.V. All rights reserved.