Model catalysts consisting of Au and Ag clusters of varying size have been prepared on single crystal TiO2(110) and ultra-thin films of TiO2, SiO2 and Al2O3. The morphology, electronic structure, and catalytic properties of these Au and Ag clusters have been investigated using low-energy ion scattering spectroscopy (LEIS), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) and spectroscopy (STS) with emphasis on the unique properties of clusters <5.0 nm in size. Motivating this work is the recent literature report that gold supported on TiO2 is active for various reactions including low-temperature CO oxidation and the selective oxidation of propylene. These studies illustrate the novel and unique physical and chemical properties of nanosized supported metal clusters.