Tungsten oxide (WO3) thin films have been extensively studied for their interesting physical properties and a variety of potential applications in electrochromic devices. In order to explore the possibility of using these in electrochromic devices, a preliminary and thorough study of the optical properties of the host materials is an important step. Based on this, the influence of annealing temperature on the structural, surface morphological, optical and electrochromic properties has been investigated in the present work. The host material, WO3 films, has been prepared from an ethanolic acetylated peroxotungstic acid sol containing 5 wt.% oxalic acid dehydrate (OAD) by sol-gel technique. The monoclinic structure and textured nature change of the films with the temperature increasing have been investigated by X-ray diffraction analysis. The surface morphology evolution of the films has been characterized by SEM. The shift in absorption edge towards the higher wavelength region observed from optical studies may be due to the electron scattering effects and the optical band filling effect that reveals the crystallization of the film. The amorphous film shows better optical modulation (Delta T = 76.9% at lambda = 610 nm), fast color-bleach kinetics (t(c) similar to 4 s and t(b) similar to 9 s) and good reversibility (Q(b)/Q(c) = 90%), thereby rendering it suitable for smart window applications. (C) 2009 Elsevier B. V. All rights reserved.