Electronic structures of gold nanoparticles supported on TiO2 have been investigated using the electron holography method, scanning tunneling microscopy, and first-principles calculations By the electron holography method, the dependence of the mean inner potential of the gold nanoparticles on TiO2 on the size of particles has been found. Mean inner potential is given as the zero-order Fourier coefficient of the crystal potential. As the size of nanoparticles becomes smaller than 5 nm, their mean inner potentials become larger than the bulk value. By the scanning tunneling microscopy, the local barrier height, which is the apparent tunneling barrier height between the tip and sample, and energy gap were measured on the gold particles supported on the rutile TiO2(1 1 0) surface-When the particles become smaller than 0.4 nm in height, the local barrier height starts to decrease and the particles change from metal to nonmetal. By the first-principles calculations, the electronic structures and the charge transfers of the gold layer adsorbed on the rutile TiO2(1 1 0) surface have been examined and the effects of the surface stoichiometry have been investigated. The gold atoms are adsorbed on the non-stoichiometric, Ti-rich and O-rich, surfaces stronger than on the stoichiometric surface. The electronic structures of the gold nanoparticles depend on the size of particles and the stoichiometry of the TiO2 surface. All these experimental and theoretical results complementarily provide valuable insight into the origin of the catalytic activity of the Au/TiO2 catalysts. (c) 2005 Elsevier B.V. All rights reserved.