Understanding liquid-water transport phenomena inside operating proton exchange membrane fuel cells PEMFCs can only be achieved by a basic understanding of gas diffusion layer GDL wetting properties. This requires measuring and quantifying the combined properties of different GDL materials and layers. These properties include surface energy, liquid absorption, internal contact angle, equilibrium contact angle, and capillary constant. The stateof-the-art microporous layer MPL and GDL substrate configuration that is nearly ubiquitously used has been adopted without a basic understanding of these fundamental properties. This lack of understanding has created poor comprehension of the relationship between PEMFC operating variables most notably inlet relative humidity RH, temperature, and current density and GDL designs. Science applied to the GDL component design helps enable PEMFCs capable of running under wet and dry operating conditions. PEMFC GDLs have a complex wetting behavior due to the constituent materials from which they are made. 1-3 GDLs are typically composed of two distinct layers, a substrate of graphitized carbon fibers and an MPL coated onto one side. The macroporous substrate layer consists of a carbon-fiber matrix and a partial fluoropolymer coating of the fibers with a typical void volume of 75‐85%. The MPL consists of carbon black particles mixed with a fluoropolymer and has a smaller porosity 40 to 50% and a mean pore size than the GDL substrate. The MPL forms a penetrating intermediate layer with the fiber matrix, creating a trilayer structure in practice. GDLs participate in most functions of an operating PEMFC, including reactant distribution to the electrode layers, removal of cathode product water, conduction of electrons, and absorption of compressive loads. Quantification of surface wetting properties of GDLs is important for several reasons: i better understanding of the mechanisms of the two-phase transport of liquid water, ii improved understanding of fuel cell performance with different operating conditions, iii fundamental understanding of the nanoscale interactions of liquid water with the heterogeneous pore surface of GDLs, and iv more sophisticated mathematical model development by providing better capillary pressure and pore-size distribution PSD inputs. Recent research undertaken to enhance the understanding of liquid-phase transport includes capillary pressure Pc vs liquid saturation SL measurements 4-8 and pore-network modeling 9-12 of GDLs. Although these approaches are different from that taken in this work, they represent important steps in the understanding of the inter-related effects of surface chemistry and pore structure. Gostick et al. measured Pc vs SL curves for a spectrum of GDL materials for both the total and hydrophilic PSDs. These data were obtained using the method of standard porosimetry and correlated with Hg porosimetry for the total PSDs. Capillary-pressure data of the GDL substrates and MPLs were separately characterized using a Leverett J-function.