Silica-supported cobalt (6 wt%) catalysts were prepared by pore volume impregnation of either ethanol or aqueous cobalt nitrate solutions on calcined silica (500 and 1000 degrees C), and characterized by infrared, hydrogen chemisorption, thermogravimetry-mass spectroscopy (TG-MS), and X-ray diffraction (XRD). XRD and TG show that the main cobalt phase is Co3O4 (above 90%) after calcination in vacuo to 300 degrees C. The influence of the calcination temperature of silica on the particle size of Co3O4 is not significant. Smaller Co3O4 crystallites and more Co-SiO2 interaction species were obtained when catalysts were prepared with ethanol instead of aqueous solution. In addition, ethoxyl groups were formed on the silica surface with a surface density of 0.17 to 0.09 ethoxyls/nm(2) depending on the calcination temperature of the pretreated silica. The decrease in Co3O4 crystallite size on ethanol impregnation is attributed to the presence of surface ethoxyls which might hinder the aggregation of Co3O4 during its formation from the thermal decomposition of cobalt nitrate. As a consequence, a larger percentage dispersion of cobalt metal was obtained after reduction at 400 degrees C. TG-MS results showed that ethoxyl groups were decomposed into ethylene in the temperature range 500-600 degrees C in He or H-2. Hence, ethoxyl groups might affect the sintering process of the cobalt metal during reduction as well.