Surfaces of bulk platinum and unsupported (Vulcan-free) Pt nanoparticles, that are modified and stabilized with such Keggin type heteropolyacids of molybdenum and tungsten as H3PMo12O40 and H3PW12O40, have been characterized using cyclic voltammetry, FTIR (by reflectance), as well as transmission and scanning electron microscopies. The presence of the polyoxometallate monolayer on platinum results in the partial suppression of the interfacial formation of PtOH/PtO oxides. Both molybdates and tungstates seem to interact with Pt surface via their corner oxygen atoms. The existence of spacious, largely hydrated, polyoxometallate monolayers on platinum does not block access of reactant (oxygen) to the catalytic Pt sites. The electrocatalytic properties of H3PMo12O40 and H3PW12O40 modified Pt nanoparticles towards reduction of oxygen in acid medium have been examined and compared using rotating ring-disk voltammetry. Reactivity of Pt-free H3PMo12O40 and H3PW12O40 with respect to reduction of hydrogen peroxide has also been considered. Our results clearly show that modification of Pt nanoparticles with PW12 (but not with PMo12) results in the enhancement of the electrocatalytic reduction of oxygen. (c) 2007 Elsevier Ltd. All rights reserved.