The kinetics of oxygen electroreduction have been studied on a smooth platinum electrode coated with Nafion(R) in concentrated 85% H3PO4 The effects of Nafion(R) coatings of different thickness on O-2 electroreduction at a smooth Pt rotating disk electrode with 85% phosphoric acid as the bulk electrolyte were examined. The kinetic current increases with increasing Nafion(R) film thickness while the diffusion limiting current decreases with increasing Nafion(R) film thickness. A O-2 concentration profile model for the Pt/Nafion(R)/bulk electrolyte has been established, and this model can be used to explain the O-2 reduction polarization results. The performance of Nafion(R)-modified, high surface area Pt/carbon air cathodes for use in the H-2-air concentrated phosphoric acid fuel cell was also studied.