This paper reports on the original and promising electrocatalytic properties of thiol-capped platinum nanoparticules toward O-2 reduction in acidic medium. The nanoparticles were initially functionalized by 4-mercaptoaniline, and were further derivatized with 2-thiophenecarbonyl chloride leading to high stabilization of the organic crown. Ultrathin films of these particles were built up by the LB technique on gold. Their electrochemical behavior was studied. These assemblages were also characterized by X-ray photoelectron spectroscopy before and after oxygen reduction experiments. An electrocatalytic activity was established without any activation treatment. Our results show that the organic crown of the nanoparticles does not prevent their electrochemical activity. (C) 2004 The Electrochemical Society.