A novel water porosimeter and its use in determining the capillarity of gas diffusion layers are described. It is found that. in accordance with the Washburn equation, the pressure required to force water into the gas diffusion layer depends on the cosine of the contact angle of water with the surface of the pore. Negative pressure is required to withdraw water from the gas diffusion layer, even when the surface is hydrophobic. The negative pressure required is found to be independent of surface contact angle. It is shown that the performance of gas diffusion layers in an operating fuel cell can be qualitatively predicted from the capillary pressure curves obtained. The advantages of the use of water porosimetry over the use of either mercury porosimetry or porosimetry using wetting fluids are discussed. (C) 2008 Elsevier B.V. All rights reserved.