The relationship of capillary pressure to liquid saturation for the water-air fluid pair in two different types of gas diffusion media (GDM) used in polymer electrolyte membrane fuel cell (PEMFC) electrodes is elucidated. It is experimentally demonstrated that GDM samples with and without treatment with poly(tetrafluoroethylene) (PTFE) ubiquitously display permanent capillary pressure hysteresis. Water does not imbibe spontaneously into a dry GDM, neither is it ejected spontaneously from a water-saturated GDM. Rather, positive displacement pressure is required to force both water and air into GDMs, whereas the main effect of adding PTFE is to increase the amount of work required for forcing water into the GDM. and to decrease the work required for water removal. Irrespective of PTFE content, the GDM samples tested are generally shown to behave as materials of intermediate (neutral) wettability. The US Bureau of Mines (USBM) wettability index nevertheless shows that water is the preferentially non-wetting phase in PTFE-treated GDMs and the preferentially wetting phase in untreated GDMs. Water-air capillary pressure curves are found to depend on sample thickness, clearly demonstrating that finite size effects are important. Finally, compression of the GDM is found to increase the capillary pressures for water injection and decrease the capillary pressures required for water withdrawal. These results should aid the design of GDMs with improved water management properties and the modeling of PEMFC electrodes in general. (C) 2009 Elsevier B.V. All rights reserved.