Gold nanoparticles were synthesized in a water-in-oil microemulsion. After the synthesis, the nanoparticles were cleaned by depositing a PbO2 film in 0.1 M NaOH + 1 mM Pb(II). The characterization of the nanoparticies was carried out by recording the lead upd voltammetric profile in the same solution. An estimation of the size of the different facets present in the nanoparticles can be made by comparing the lead upd profile of the nanoparticles with those obtained for the gold single crystal basal planes. The effect of different species added to the water phase of the microemulsion on the surface structure of the nanoparticles was also examined. The nanoparticles synthesized in the presence of iodide show a higher ratio of (1 0 0) and (1 1 1) facets, whereas (1 0 0) and (1 1 1) facets are almost absent for those synthesized in the presence of sulfide. Finally, the electrocatalytic behavior of the nanoparticles for the oxygen reduction reaction in 0.1 M NaOH was studied using a rotating ring-disk arrangement. The highest catalytic activity is obtained for the nanoparticles synthesized in the presence of iodide. These results were analyzed in the light of those obtained for the same reaction on gold single crystal electrodes. (C) 2003 Elsevier B.V. All rights reserved.