By electrochemically controlling the structure of the surface aggregates, the grain microstructure has been optimized to yield mesoporous thin films of tungsten oxide (WO 3 ) at the electrode-electrolyte interface in a peroxotungstate sol in the presence of a structure-directing agent (Triton) at room temperature. Apart from the dominant ultrafine nanocrystallites and pores (5-10 nm), well-developed abutting grains (25-100 nm) and nanofibrils also constitute an integral part of the film matrix. X-ray photoemission spectra reveal the as-deposited film (WO 3-x ) to be constituted by a high proportion of W 6+ states with a low oxygen deficiency (x = 0.02). A relatively high W 5+ content in the film, upon intercalation of 18 mC cm -2 charge translates into a large coloring efficiency (η VIS ∼ 70 cm 2 C -1 ) and transmission modulation. At a lithium intercalation level of 22 mC cm -2 , in addition to W 5+ and W 6+ states, the film also comprises of W 4+ states. The extremely fast color-bleach kinetics (3 and 2 s, respectively, for a 50% change in transmittance) shown by the as-deposited WO 3 film are repercussions of the mesopore morphology, the multiple nanostructures and the sixfold channels of its hexagonal modification. The film shows a high cycling stability as the switching times do not show any significant decline even after 3500 repetitive cycles. Coloration efficiency over the solar and photopic regions and current density for lithium intercalation for the as-deposited film are superior to that observed for the films annealed at 100, 250 and 500 °C. The abysmal electrochromic response of the annealed films is a consequence of surface defects like cracks and uncontrolled densification and pore shrinkage. © 2007 Elsevier B.V. All rights reserved.