This paper explores the effect and siting (location) of Nation on Pt/C as exists in a PEM fuel cell catalyst layer. The addition of 30 wt% Nation on Pt/C (Nfn-Pt/C) resulted in a severe loss of BET surface area by filling/blocking the smaller pore structures in the carbon support. Surprisingly, the presence of this much Nation appeared to have only a minimal effect on the adsorption capability of either hydrogen or CO on Pt. However, the presence of Nation doubled the amount of time required to purge most of the gas-phase and weakly-adsorbed hydrogen molecules away from the catalyst during hydrogen surface concentration measurements. This strongly chemisorbed surface hydrogen was determined by a H(2)/D(2) switch and exchange procedure. Nation had an even more pronounced effect on the reaction of a larger molecule like cyclopropane. Results from the modeling of cyclopropane hydrogenolysis in an idealized pores suggest that partial blockage of only the pore openings by the Nation for the meso-macropores is sufficient to induce diffusion limitations on the reaction. The facts suggest that most of the Pt particles are in the meso-macropores of the C support, whereas Nation is present primarily on the external surface of the C where it blocks significantly the micropores but only partially the meso-macropores. (C) 2011 Elsevier B.V. All rights reserved.