Assemblies of polyacrylate-capped Pt nanoparticles (< d > = 2.5 +/- 0.6 nm) in poly (diallyldimethylammonium chloride) are reported to be electrocatalytically active for key charge-transfer reactions, viz., hydrogen evolution, hydrogen oxidation, and oxygen reduction and can thus serve as model nanostructured electrodes for studying electrocatalysis at well-characterized solution-prepared Pt and other metal nanoparticles. These results indicate that catalytic adsorption sites for the O-2 reduction and H-2 oxidation reactions are not blocked at the surface-modified nanoparticles, and that charge and mass transport supporting these electrochemical reactions are feasible in the films. Comparison is made with an electrochemically activated polycrystalline Pt electrode. (c) 2006 The Electrochemical Society.