Different types of planar Au/TiO(2) model catalysts are produced on TiO(2)(110) single-crystal substrates and thin TiO(2) films on Ru(0001) by physical vapor deposition of gold under ultrahigh-vacuum (UHV) conditions or by micelle-based chemical routes. Both the Au nanoparticles and the support are characterized by surface-science-based methods (such as atomic force microscopy and X-ray photoelectron spectroscopy) as well as by transmission electron microscopy. Finally, the activity of the model catalysts in the CO oxidation reaction is analyzed in a microflow reactor. Au/TiO(2)(110) model catalysts with a stoichiometric TiO(2)(110) support exhibit only a low catalytic activity compared to those with a reduced crystal and Au/TiO(2) model catalysts with thin TiO(2) films on Ru(0001) as a substrate. The possible influence of Ti interstitials in the reduced TiO(2)(110) substrates on the CO oxidation activity is discussed.