A potentiostatic electrochemical procedure employing an ethanolic solution of peroxotungstic acid yielded tungsten oxide (WO3) films specifically for transmissive electrochromic devices (ECDs) such as "smart windows". The structure-property correlation for the as-deposited thin films of WO3 and the films annealed at different temperatures (60, 100, 250 and 500 °C) is described. While the as-deposited film comprises of a small volume fraction of nanocrystals, the films annealed at 60 and 100 °C are X-ray amorphous, the 250 °C film is triclinic and the 500 °C film is characterized by mixed triclinic and hexagonal crystalline phases. Scanning electron micrographs (SEMs) clearly reveal the presence of nanograins/ crystallite aggregates in the heat-treated films. Coloration renders the 250 °C film amorphous and reduces the crystallite size in the as-deposited and 500 °C films. Thermal analysis provides information pertaining to the critical temperatures at which dehydration, peroxo group decomposition and crystallization occur. Fourier transform infrared (FTIR) spectroscopic data show that while the as-deposited WO3 film is composed of a peroxopolytungstate ion network with water molecules acting as bridging groups, the crystalline film annealed at 500 °C shows absorption bands characteristic of the W-O-W network (polytungstate ions linked through oxygens). Coloration efficiency (CE; η) and transmission modulation (ΔT) are slightly higher for the amorphous 60 °C film when compared with the nanocrystalline as-deposited film in the 450<λ<850 nm range, and these properties deteriorate at temperatures ≥100 °C. However, switching times between the colored and bleached states and ion storage capacities show a systematic and significant decrease with increasing annealing temperature. High transmission modulation (67.5% at 632.8 nm), fast bleaching kinetics (∼5 s), high ion storage capacity (22 mC cm-2) and a coloration efficiency of 60.5 cm2 C-1 at 632.8 nm observed for the as-deposited film render it to be a suitable candidate for electrochromic window applications. An operational lifetime of more than 104 cycles has been tested successfully for the as-deposited film against a platinum sheet as an auxiliary electrode. © 2004 Elsevier B.V. All rights reserved.