The adhesion, shape, and electronic structure of gold particles supported on TiO2(110) have been determined by density-functional theory calculations, to gain more insight into the catalytic activity of small supported gold particles. These calculations show that gold particles do not bind to a perfect TiO2 surface, but have a binding energy of about 1.6 eV/defect on an oxygen vacancy in TiO2. This results in an indirect effect of the support material on the catalytic activity: The distribution and dynamics of the oxygen vacancies determine the dispersion and shape of the gold particles, which in turn affect the catalytic activity. A theoretical analysis of the gold particle shape reveals that it is flat, about 3-4 layers for a gold particle of 3 nm diameter. The calculated flat geometry is in good agreement with electron microscopy and EXAFS measurements of a high surface area Au/TiO2 catalyst containing 2.4 wt% gold. (C) 2004 Published by Elsevier Inc.