To fabricate electrocatalytic network films containing Pt nanoparticles, the ability of a Keggin-type polyoxometallate, phosphododecatungstate (PW12O403-), to form stable anionic monolayers on solid surfaces is explored. Three-dimensional assemblies on electrode (glassy carbon or platinum) surfaces are grown using the layer-by-layer method involving repeated alternate treatments in the solution of PW12O403- (or in the colloidal suspension of polyoxometallate-protected Pt-nanoparticles) and in the solution of monomer (e.g., anilinium) cations. In the resulting structured (organic-inorganic) films, the layers of negatively charged polyoxometallate, or polyoxometallate-protected (stabilized) Pt-nanoparticles, interact electrostatically with the ultra-thin layers of such a positively charged conducting polymer as polyaniline. Consequently, physicochemical properties of organic conducting polymers, and reactivities of inorganic polyoxometallate and/or noble metal particles can be combined. The modification of Pt nanoparticles by adsorbing monolayers of phosphododecatungstate tends to activate them towards efficient electrocatalytic reduction of oxygen in acid medium. (C) 2005 Elsevier Ltd. All rights reserved.