The structure of Au heterogeneous catalysts on the anatase form of TiO2 has been characterized by high-resolution electron microscopy (HREM) and quantitative Z-contrast scanning transmission electron microscopy (STEM). These materials are prepared by deposition of highly monodisperse Au-13[PPh3](4)[S(CH2)](11)CH3](4) (8-angstrom diameter) ligand-protected clusters on anatase, followed by reaction with ozone or a rapid oxidative thermal treatment to remove the ligands. The materials obtained differ markedly in each case. For the thermal treatment at 400 C in air, the supported particles grow to an average size of 2.7 nm (+/- 0.6 nm) in diameter, and the larger particles in the distribution are found to adopt a spherical geometry. Particle growth is greatly inhibited when ozone is used to remove the ligands (average diameter 1.2 +/- 0.5 nm). These particles assume a more oblate geometry, consistent with a truncated hemispherical shape. It was found that subsequent thermal treatments of the ozone-derived supported nanoparticles at 400 degrees C in air did not induce additional growth, indicating that sintering is strongly affected by the particle-support interactions developed by the ozone-based low-temperature ligand removal step. These materials exhibit catalytic activity and high stability for the oxidation of CO at elevated temperatures, with the level of activity dependent on catalyst preparation. (c) 2006 Elsevier Inc. All rights reserved.