A sensitive method is presented for studying adsorption of gaseous species on metal surfaces in vacuum by attenuated total internal reflection Fourier transform infrared spectroscopy (ATR). The method is illustrated by CO adsorption experiments on silica supported Rh nanoparticles. An experimental setup and a procedure are described in detail to obtain a sensitivity of reflectance change of similar to 5 X 10(-5) absorbance units. Here, a silicon ATR crystal with a 50 nm layer of hydroxylated silica acts as the support for the Rh nanoparticles. These particles are easily prepared by spincoat impregnation from a RhCl3 solution followed by H-2 reduction. X-ray photoelectron spectroscopy before and after reduction shows that rhodium is reduced to Rho and that all chlorine is removed. Atomic force microscope images the distribution of the particles, which are 3-4 nm in height. When the crystal is exposed to pressures up to 1 mbar of CO, a gas which is inert to the silica support, the stretch vibration of linearly adsorbed CO on the Rh nanoparticles is detected at 2023 cm(-1), while no bridged CO or geminal dicarbonyl species can be distinguished. The minimum detectable coverage is estimated similar to 0.005 CO per nm(2) substrate area or similar to 5 X 10(-4) ML. (c) 2006 American Vacuum Society.