The CO oxidation behaviour of planar Au/TiO 2 model catalysts was studied by a combination of kinetic measurements and in-situ IR measurements under realistic pressure and temperature conditions (p=20mbar, T=80°C). During reaction in a CO/O 2 mixture, the model catalysts deactivated significantly. Adding small amounts of H 2 O (1000ppm) to the reactive CO/O 2 gas mixture, the catalyst activity increased markedly, by at least one order of magnitude. Furthermore, in-situ IR measurements show a decrease of the amount of adsorbed by-products of the CO oxidation, and a change of the predominant by-product from surface carbonate to surface formate. Possible reasons for the deactivation are discussed and the effect of H 2 O addition is compared to results of similar studies on more realistic dispersed catalysts.