The structure of Pt nanoparticles and the composition of the catalyst-Nafion films strongly determine the performance of proton exchange membrane fuel cells. The effect of Nation content in the catalyst ink, prepared with a commercially available carbon-supported Pt, in the kinetics of the hydrogen oxidation reaction (HOR), has been Studied by the thin layer rotating disk electrode technique. The kinetic parameters have been related to the catalyst nanoparticles Structure, characterized by X-ray diffraction and high-resolution transmission electron microscopy. The size-shape analysis is consistent with the presence of 3D cubo-octahedral Pt nanoparticles with average size of 2.5 nm. The electrochemically active surface area, determined by CO stripping, appears to depend on the composition of the deposited Pt/C-Nafion film, with a maximum value of 73 m(2) g(pt)(-1) for 30 wt.% Nafion. The results of CO stripping indicate that the external Pt faces are mainly (100) and (111) terraces, thus confirming the cubo-octahedral Structure of nanoparticles. Cyclic voltammetry combined with the RDE technique has been applied to study the kinetic parameters of HOR besides the ionomer resistance effect on the anode kinetic Current at different ionomer contents. The kinetic parameters show that H-2 oxidation behaves reversibly with an estimated exchange Current density of 0.27 mA cm(-2). (c) 2009 Elsevier B.V. All rights reserved.