Using high-resolution transmission electron microscopy, it was established that WO3 thin films (d = 190 nm) prepared by the sol-gel method exhibit a nanocrystalline ReO3-type structure already at curing temperatures not exceeding 180 degrees C. Electron diffraction studies showed no additional crystalline or amorphous hydrated phases WO3 . nH(2)O in samples obtained at this temperature. This result is important with respect to the kinetic behavior and aging properties of electrochromic devices. As detected by X-ray diffraction, a crystalline phase appeared when the thin films were heated at 325 degrees C for 1 h. A comparison of the electron diffraction pattern of the latter with the one of the materials cured at 180 degrees C showed growth of the crystals in size, but no changes in structure or ordering.