The chemical and electronic properties of surfaces to a large extent are controlled by defects. Defects facilitate the adsorption of gases and serve as reactive sites for chemical reactions. Furthermore, surface defects are a key to the nucleation, growth, and stability of metal clusters on metal oxide surfaces. This paper summarizes recent studies to assess the role of defects on Au cluster nucleation, growth, and stability on SiO2 and mixed TiOx-SiO2 thin films. For a SiO2 thin film, Au clusters sinter at elevated temperatures and pressures; however, introduction of defects on a SiO2 surface as TiO, islands or as substitutional Ti dramatically decreases the rate of An cluster sintering. (C) 2004 Elsevier B.V. All rights reserved.