Silica-supported rhodium catalysts prepared from nitrate and chloride precursors were tested in the CO hydrogenation reaction. Similar reaction rate and selectivities to the different product families were found for both catalysts, however the ex-chloride catalyst showed a lower 1-olefin/n-paraffin ratio. Characterisation by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), H-2 chemisorption and Fourier transform infrared spectroscopy (MR) of CO chemisorbed did not evidence disparate features between ex-nitrate and ex-chloride samples. However, a much higher H-2 desorption was found in the ex-chloride catalyst according to the hydrogen temperature-programmed desorption (TPD) studies. Residual chloride species seems to be involved in the H-2 adsorption on silica enhancing the spillover of hydrogen from metal particles to the silica support. It is suggested that spilt-H atoms can create active sites on the silica surface where olefins can be hydrogenated to the corresponding paraffins. (C) 2003 Elsevier Science B.V. All rights reserved.