We studied the effect of Nation on the formic acid oxidation reactivity of three unsupported nanoparticle catalysts: Pt black, Pt/Ru black, and Pt/Pd. The catalysts were prepared by physisorption deposition of the nanoparticles on Au both with and without Nation as an adhesive. The results indicate that Nafion lowers both the apparent surface area and the formic acid oxidation current by blocking surface sites. Surface areas obtained with Nation binding agent were suppressed by 13, 8, and 22%, while formic acid oxidation currents were attenuated by 40, 13 and 27% for Pt black, Pt/Ru black and Pt/Pd, respectively. When comparing the effects of Nafion on the surface areas, and comparing these results to the respective voltammograms, Nation displays some selective site blocking effects as well. (C) 2003 Elsevier Science B.V. All rights reserved.