Thin films of WO3 for electrochromic window applications have been potentiostatically electrodeposited from a precursor sol of acetylated peroxotungstic acid. Although the film annealed at 100°C is amorphous to X-rays, transmission electron microscopy (TEM) findings reveal that the film is not only nanostructured but is also characterized by localized regions of amorphous phase and microcrystals of the hexagonal structure. The porous and the granular structure of the film is also illustrated by scanning electron microscope (SEM) images at different magnifications (≥5 K). The modes of coordination of water molecules and oxygens in the WO3 film matrix are clearly evident from Fourier transform infrared (FTIR) spectroscopic data. The suitable choice of organic additive and appropriate annealing conditions allowed the tailoring of the microstructure of the film to impart fast bleaching kinetics (tb=8 s) and high values of transmission modulation (ΔT=68%), coloration efficiency (CE; η=62 cm2 C -1) and ion storage capacity (ISC>20 mC cm-2) at λ=632.8 nm. Rapid rates of diffusion (DLi>10-10 cm2 s-1) and a high ion mobility (μLi> 10-7 cm2 V-1 s-1) observed for the reversible lithium insertion-extraction process affirms the suitability of the film for smart windows. Cyclic voltammetric studies and multiple-step chronoamperometry showed that insignificant changes are observed in these electrochemical and electrochromic properties of the film even after cycling it 104 times in a 1 M LiClO4-PC electrolyte. It is evidenced from the present investigations that the microstructure of the film predominantly controls its electrochromic performance. © 2004 Elsevier B.V. All rights reserved.