Abstract We have compared the adsorption properties of small Au n ( n = 1–8) nanoparticles on the defect-free (stoichiometric) and defective (partially reduced) brookite TiO 2(210) and anatase TiO 2(101) surfaces using density functional theory calculations. The interaction between Au atoms and anatase TiO 2(101) was determined to be quite weak and small Au n particles grown at defects (O vacancies) prefer extended 2D structures. By contrast, dispersion and 3D configurations appear to be favored at brookite TiO 2(210) for Au n nanoparticles due to their strong interaction. Calculations of CO oxidation at Au n ( n = 6–8) particles supported at defective brookite TiO 2(210) show that occurrence of protruding low-coordinated Au atoms is essential for favorable CO adsorption and subsequent reaction with O 2. In particular, the configuration of the Au n nanoparticles can determine the energetics in the formation of active Au atoms, and their mobility also affects the reaction between CO and O 2 (or O).