Recent research in heterogeneous catalysis, especially on size-selected model systems under UHV conditions and also in realistic catalytic environments, has proved that it is necessary to think in terms of the exact number of atoms when it comes to catalyst design. This is of utmost importance if the amount of noble metal, gold in particular, is to be reduced for practical reactions like CO oxidation. Here it is shown that on TiO2 only Au6 and Au7 clusters are active for CO oxidation which holds for the single crystal, thin films, and titania clusters deposited on HOPG. Size-selected cluster deposition and TPD methods have been employed to investigate the CO oxidation activity of Aun/TiO2 systems which are compared to recent results reported by Lee et al. to form a consistent picture in which only two species can be regarded as "active". The efficiency of investigated Aun/(TiO2)93/HOPG composite materials is attributed to carbon-assisted oxygen spillover from gold to support particles and across grain boundaries.