Palladium nanoparticles protected by polyvinylpyrrolidone (PVP) have been synthesized by the reduction-by-solvent method and deposited onto alumina by the wet impregnation method. The metallic colloid has been left for different times (aging times) at room temperature and under open bench conditions before deposition of the particles on the support. The colloids have been analyzed by TEM, and the catalysts prepared have been characterized by TEM, TG-MS, XPS and IR spectroscopy. The catalytic activity of the nanoparticles has been tested in the preferential oxidation (PrOx) of CO reaction, which consists in CO removal from H-2-rich streams. The reaction was conducted up to 200 degrees C and with a gas composition similar to that coming out from a reformer (2% CO, 2% O-2, 30% H-2, He). The aging time of the colloidal suspensions, i.e. the time elapsed between colloids preparation and deposition, does not have an influence over the nanoparticles size or shape. However, this parameter has shown a great influence on the catalytic behavior of the metal nanoparticles, affecting both their catalytic activity and selectivity. The catalyst impregnated with the immediately prepared nanoparticles is the one showing the best catalytic properties in this work. The amount of polymer anchored to the particles surface and its interaction with the metal surface has been found to be responsible for these observations. (C) 2012 Elsevier B.V. All rights reserved.