Colloidal Co nanoparticles with sizes in the 3-8 nm range were obtained by thermal decomposition of Co-2(CO)(8) in the presence of ligands and impregnated on SiO2 to prepare SiO2-supported Co nanocatalysts. The catalysts showed activity for the steam reforming of ethanol with higher values for smaller Co particles. H-2 adsorption results and Fourier transform infrared spectroscopy of adsorbed CO suggested that the fraction of accessible Co sites also depended on the synthesis conditions. Precipitation of the Co nanoparticles with methanol instead of ethanol before impregnation had a positive effect on the density of accessible Co sites to catalysis: similar result was verified by increasing the thermal treatment temperature under H-2 flow before the reaction. Based on the distribution of products with temperature of reaction, a mechanism for steam reforming of ethanol on SiO2-supported Co nanocatalysts is suggested. (C) 2009 Elsevier B.V. All rights reserved.