Nafion (R) ionomer content of the cathode catalyst-layer of a polymer electrolyte fuel cell (PEFC), made by the "decal" hot pressing method, has been investigated for its effect on performance and structure of the membrane electrode assembly (MEA) Varying Nafion (R) content was shown to have an effect on performance within the entire range of polarization curves (i e kinetic, ohmic. and mass-transport regions) as well as on the structure AFM analysis shows the effect of Nation on the dispersion of carbon aggregates Further analysis using TEM demonstrates the effect of Nation on both the dispersion of carbon aggregates and the distribution and thickness of the Nation ionomer films surrounding the catalyst/carbon aggregates The MEA structure change correlates well with the MEA performance on both kinetics and mass-transport region The determining factors on the performance of MEA are the interfacial zone (between the ionomer and catalyst particle), the dispersion of catalyst/carbon aggregates and the distribution/thickness of Nation films An optimized Nafion (R) content in the range of 27 +/- 6 wt.% for the cathode was determined for an E-TEK 20% Pt3Cr/C catalyst at a loading of 0 20 mg Pt/cm(2) (C) 2010 Elsevier Ltd. All rights reserved.