Two supported Ru catalysts were prepared by the chemical vapor deposition of Ru-3(CO)(12) on MgO and SiO2 (MOCVD). TEM, XRD, and static H-2 chemisorption measurements confirmed that the Ru particle size was about 2 nm on both supports. Using in situ DRIFT (diffuse reflectance infrared Fourier transform) spectroscopy at atmospheric pressure it was found that the adsorption of CO on the reduced samples is clearly influenced by the supports whereas the adsorption of CO on the oxidized Ru catalysts is essentially independent of the support. O-2 chemisorption measurements showed that a thin RuO2 surface layer was formed on both catalysts under oxidizing conditions at room temperature. The observed C-O stretching frequencies were found to be in good agreement with HREELS and LEED data reported for the RuO2(1 1 0) single crystal surface. The catalytic activity was assessed under high-vacuum conditions using the TAP (temporal analysis of products) reactor by co-feeding CO and O-2. These conditions ensured that heat and mass transfer limitations were absent. Both supported Ru catalysts were found to be highly active and stable under the CO oxidation conditions even down to room temperature. The deactivation of the catalysts observed at room temperature was reversible and independent of the support. The turnover frequencies (number of CO2 molecules per metal surface site per second) derived from steady-state measurements are in good agreement with data reported for the RuO2 (1 1 0) single crystal surface under UHV conditions. Based on the results of the DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) and the kinetic measurements supported RuO2 is identified as the catalytically active phase. In addition, the turnover frequencies are in good agreement with data reported for Ru/SiO2 at atmospheric pressure. Thus, both the materials and the pressure gap were bridged successfully. (C) 2003 Elsevier B.V. All rights reserved.