In this investigation, the effect of gold nanocrystals on the electrochromical properties of sol-gel Au doped WO3 thin films has been studied. The Au-WO3 thin films were dip-coated on both glass and indium tin oxide coated conducting glass substrates with various gold concentrations of 0, 3.2 and 6.4 mol%. Optical properties of the samples were studied by UV-visible spectrophotometry in a range of 300-1100 nm. The optical density spectra of the films showed the formation of gold nanoparticles in the films. The optical bandgap energy of Au-WO3 films decreased with increasing the Au concentration. Crystalline structure of the doped films was investigated by X-ray diffractometry, which indicated formation of gold nanocrystals in amorphous WO3 thin films. X-ray photoelectron spectroscopy (XPS) was used to study the surface chemical composition of the samples. XPS analysis indicated the presence of gold in metallic state and the formation of stoichiometric WO3. The electrochromic properties of the Au-WO3 samples were also characterized using lithium-based electrolyte. It was found that doping of An nanocrystals in WO3 thin films improved the coloration time of the layer. In addition, it was shown that variation of Au concentration led to color change in the colored state of the Au-WO3 thin films. (C) 2009 Elsevier B.V. All rights reserved.