Photochromism (PC) and electrochromism (EC) of transition metal oxides have received much attention because of their potential technological applications in displays. Consequently, many kinds of transition materials oxides have been investigated being WO3 and MoO3 the most studied for such purposes. In this work, the spectral variations of the optical transmission under polarization or illumination are reported for thin films of doped and undoped tungsten trioxide prepared by the sol-gel process. Tungsten trioxide was deposited onto glass by the dip-coating technique, with a film thickness of about 450 nm. The glass/ WO3 films were illuminated with sunlight for 60 min in order to investigate the photochromic sensitivity of the films. The influence of niobium, tantalum, titanium and zirconium on the photochromic properties of WO3 was analyzed. X-ray diffraction studies showed that the WO3 films were amorphous, when thermally treated at temperatures ranging from 100 to 150degreesC. (C) 2003 Elsevier B.V. All rights reserved.