Model catalysts of Au clusters supported on TiO2 thin films were prepared under ultra-high vacuum (UHV) conditions with average metal cluster sizes that varied from similar to 2.5 to similar to 6.0 nm. The reactivities of these Au/TiO2 catalysts were measured for CO oxidation at a total pressure of 40 Torr in a reactor contiguous to the surface analysis chamber. Catalyst structure and composition were monitored with Auger electron spectroscopy (AES) and scanning tunneling microscopy and spectroscopy (STM/STS). The apparent activation energy for the reaction between 350 and 450 K varied from 1.7 to 5 kcal/mol as the Au coverage was increased from 0.25 to 5 monolayers, corresponding to average cluster diameters of 2.5-6.0 nm. The specific rates of reaction ((product molecules) x (surface site)(-1) x s(-1)) were dependent on the Au cluster size with a maximum occurring at 3.2 nm suggesting that CO oxidation over Au/TiO2(001)/Mo(100) is structure sensitive.