Platinum nanoparticles generated on the surface of spherical polyelectrolyte brush particles were investigated with regard to their catalytic behavior in hydrogenation reactions. No surface-active agent was needed to stabilize the platinum particles. Transmission electron microscopy demonstrated that the Pt nanoparticles are well defined with a diameter of approximately 2 nm. The catalytic activity of the immobilized Pt particles, as well as the colloidal stability of the carrier particles, were preserved over many runs. These data demonstrate that the generation and immobilization of bare metallic nanoparticles on spherical polyelectrolyte brushes generate a very robust, water-based catalyst system. Pseudo-first-order kinetics with respect to the aldehyde were observed. (c) 2006 Elsevier Inc. All rights reserved.