This is the first in a series of papers in which we introduce a validated Leverett function appropriate for SGL 24 series carbon paper fuel cell diffusion media (DM) that describes the capillary pressure as a function of liquid saturation and hydrophobic additive content. The presented relationship was derived based on the benchmark data gathered from direct measurements of drainage capillary pressure-saturation of differently engineered DM (coated with a microporous layer) tailored with various poly (tetrafluoroethylene) content (ranging from 5 to 20 wt %) at room temperature under no compression. A key feature of the relationship is that the contact angle is implicitly embedded into the hydrophobicity parameter, thereby eliminating any ambiguity regarding the selection of a representative surface contact angle. A detailed comparative analysis of the present Leverett function and the standard Leverett function with the experimental data was performed. The standard Leverett function is found to exhibit significant deviation from the experimental data, whereas the present empirical function successfully predicts the capillary pressure over the entire saturation domain. Finally, additional spontaneous capillary imbibition experiments were performed to obtain estimates for the water retention capacity of a typical fuel cell DM as a function of the hydrophobicity of bilayered DM. (c) 2007 The Electrochemical Society.