Palladium nanoparticles protected by polyvinylpyrrolidone (PVP) have been synthesized by the reduction-by-solvent method and deposited on alumina by impregnation. The prepared catalysts have been tested in the preferential oxidation of carbon monoxide (PrOx) in H-2-rich gaseous streams and the results have been compared with catalysts prepared by conventional impregnation. Pd-nanoparticles protected by PVC have good activity and outstanding selectivity towards CO oxidation (above 50% in some cases). On the contrary, the impregnated catalyst has very low selectivity (about 10%). The influence of crucial parameters such as the capping agent-to-metal ratio or the synthesis temperature of the nanoparticles on the catalytic performance of the prepared materials has been analyzed. Our results show that within the 2.5-3.5 nm region, particle size of supported nanoparticles does not seem to exert a significant influence on the studied reaction. However, the amount of capping agent seems to be crucial to prepare efficient, stable catalysts. The samples prepared show 100% CO removal at 180 degrees C when tested in the presence of CO2 and water vapour. (C) 2010 Elsevier B.V. All rights reserved.