The interfacial structure of the perfluoro-sulfonic acid ionomer and Pt-loaded carbon in a catalyst ink significantly influences the properties of the catalyst layer in polymer electrolyte fuel cells (PEFCs). This is primarily because some ionomer molecules adsorb onto the catalyst particles. The effect of Pt loading on the adsorption of the ionomer onto the surface of the particles was evaluated using contrast-variation small-angle neutron scattering (CV-SANS). Decreased density and increased thickness of the ionomer-adsorption layer were observed with increasing Pt loading on the carbon. While specific interactions between sulfonic acid groups and Pt catalyst were active, the hydrophobic attraction of the ionomer backbone to the carbon surface was the dominant force in the catalyst inks. The CV-SANS technique was useful for investigating design guidelines for the catalyst ink to improve the structure of catalyst layers.