This is the first in a series of papers in which we present state-of-the-art methods demonstrated at Case for the estimation of transport properties in gas diffusion layers (GDLs) for proton exchange membrane fuel cells (PEMFCs). Most of the methods used today for measuring wettability properties of GDLs are related to the external contact angle to water. The external contact angle however does not describe adequately capillary forces acting on the water inside the GDL pores. We show as well that the direct method of estimation of the internal contact angle using goniometry on micrographs is impractical. We propose and describe in this paper a method for estimating the internal contact angle to water and the surface energy of hydrophobic and hydrophilic gas diffusion media. The method was applied to GDLs having different contents of hydrophobic agent and carbon types. The method can be applied separately to different components of the GDL including macro-porous substrates and micro-porous layers. The uncertainty estimates using this method are usually within 3% of the measured value. (c) 2006 Elsevier B.V. All rights reserved.