Thin films of amorphous tungsten trioxide (WO3) have been thermally evaporated onto quartz substrates held at 350 K. Annealing at 723 K caused the formation of crystalline WO3. X-ray diffraction (XRD) analysis revealed monoclinic structure with lattice parameters of a = 7.311 +/- 0.05 angstrom, b = 7.603 +/- 0.05 angstrom, c = 7.713 +/-0.05 angstrom and beta = 91.137 degrees. Optical properties were investigated for both the amorphous and annealed films by using spectrophotometric measurement of transmittance and reflectance at normal incidence in the wavelength range of 200-2500 nm. The obtained spectra showed that all the films are transparent above lambda = 500 nm and the measured optical constants are independent of the film thickness. Both the refractive index, n, and absorption index, k, were determined for the amorphous and crystalline films. The optical dispersion parameters have been analysed by single oscillator model. The values of oscillator energy (E-o) and dispersion energy (E-d) were found to be 5.95 and 18.18 eV for the amorphous films and 2.92 and 11.13 eV for the crystalline films. Indirect optical transitions with corresponding energy gaps = 3.28 and 3.03 eV for amorphous and crystalline films, respectively, were obtained. The obtained results are compared and discussed. (c) 2005 Elsevier B.V. All rights reserved.