The activity and selectivity of supported metal clusters can in principle be manipulated by controlling the electronic properties of the support, as initially proposed by Schwab. To quantitatively demonstrate this effect, a series of anatase TiO2 thin films with an order of magnitude variation in the carrier concentration were grown by atomic layer deposition. The change in the TiO2 carrier concentration influences the electronic properties of supported Pt clusters, as shown by photoelectron spectroscopy. The gradual increase in the carrier concentration increases the CO oxidation rate over the Pt/TiO2 catalysts by 70% for excess CO conditions and decreases the rate by 30% for excess O-2 conditions, providing a quantitative connection between the support properties and the measured reaction rate. Density functional theory calculations and natural bond analysis show that charge injection into Pt clusters reduces the CO adsorption energy due to increased Pauli repulsion, which is consistent with the observed changes in the reaction rate. (C) 2013 Elsevier Inc. All rights reserved.