A detailed systematic study of the tungsten oxide thin films has been carried out using WO3 films after they were annealed at progressively increasing temperatures ranging from 350degreesC to 450degreesC in oxygen environments. The structural properties of the films were characterized using X-ray diffraction and Raman spectroscopy. The amorphous WO3 films remain as an amorphous phase up to 385degreesC and begin to crystallize at 390degreesC and then are completely crystallized at 450degreesC. Absorption peaks of the films are found to shift to a higher energy side with increasing annealing temperature up to 385degreesC and then shift abruptly to a lower energy as the films begin to crystallize at 390degreesC. Deconvolution of the absorption spectra shows that there are two different polaron transitions in the amorphous WO3 films. (C) 2003 Elsevier B.V. All rights reserved.