The catalytic behavior of Pt and Rh catalysts during the gas-phase oxidation of H-2-CO mixtures was studied with commercial Pt/C, Pt-Ru/C and Pt-Sn/C electrocatalysts, as well as laboratory prepared catalysts of Pt and Rh supported on various carriers (gamma-Al2O3, TiO2, TiO2(WO3 SiO2, WO3, MoO3 and carbon). The goal of this study was to assess the CO-tolerance of the catalysts, i.e. the lowering of the H-2 oxidation rate in the presence of CO, generally observed both in gas-phase and electrochemical environments. A significant drop of hydrogen oxidation rate, up to 5 orders of magnitude, was observed in the presence of CO. CO inhibition was strongly dependent on the nature of the support. The standard Pt/C electrocatalyst was the least CO-tolerant catalyst among those tested, while Pt-Sn/C, as well as Pt and Rh supported on MoO3 and TiO2-based carriers were the most active and CO-tolerant catalysts. The experimental findings indicate that CO inhibition is due to CO adsorption in linear and/or bridged form depending on the type of metal and support employed. The CO tolerance of the catalysts depends on the relative adsorption strength of CO and hydrogen on the metal surface. Simulation of CO inhibition showed that DeltaH(CO) (or the relative values of DeltaH(CO) and DeltaH(H2)) may vary up to 20-40 kJ/mol between the most and least CO-tolerant catalysts. (C) 2004 Elsevier B.V. All rights reserved.