Improvement of Pt activity for electrocatalytic reactions is possible by modifying Pt nanoparticles with other metals able to activate water. Selected examples are discussed with the electrooxidations of methanol and ethanol or the electroreduction of dioxygen. Nanoparticles electrodes of Pt-Ru (for methanol oxidation), of Pt-Sn (for ethanol oxidation) or of Pt-Cr (for oxygen reduction) supported on carbon powder can be prepared from colloidal precursors. This kind of preparation allows varying the composition and/or the structure of the electrode. The formulation of improved electrodes can be obtained after complete study of the reaction mechanism by "in situ" spectroscopic experiment. (c) 2005 Elsevier Ltd. All rights reserved.