The wetting properties of proton exchange membrane fuel cell (PEMFC) gas diffusion layers (GDLs) were quantified by surface characterization measurements and modeling of material properties. Single-fiber contact-angle and surface-energy (both Zisman and Owens-Wendt) data of a wide spectrum of GDL types are presented to delineate the effects of hydrophobic postprocessing treatments. Modeling of the basic sessile-drop contact angle demonstrates that this value only gives a fraction of the total picture of interfacial wetting physics. Polar forces contributed 10-20x less than dispersive forces to the composite wetting of GDLs. Internal water contact angles obtained from the Owens-Wendt analysis were measured at 13-19 degrees higher than their single-fiber counterparts. An inverse relationship was found between internal contact angle and both Owens-Wendt surface energy and percent polarity of the GDL. The most sophisticated PEMFC mathematical models use either experimentally measured capillary pressures or the standard Young-Laplace capillary-pressure equation. Based on the results of the Owens-Wendt analysis, an advancement to the Young-Laplace equation was proposed for use in these mathematical models, which utilizes only solid surface energies and a fractional surface coverage of a fluoropolymer. Capillary constants for the spectrum of analyzed GDLs are presented for the same purpose.