The precursor containing peroxotungstic acid (PTA) was prepared by the reaction of tungstic acid with H2O2 (30%) solution in water-ethanol mixture. This was utilized for the deposition of WO 3·nH2O film on conducting (F-doped SnO2 coated) flat glass substrate by the dipping technique. The films were cured in the temperature range of 200-300 °C in air. The optimum thickness of the film (300-500 nm) was obtained by successive operations (3-5 numbers). Films of around 500 nm thickness exhibited 60-70% transmission in the visible region. Electrochromic properties (colouration↔bleaching) of the films were studied by cyclic voltammetry (CV) using a classical three-electrode potentiostatic cell system. The cell system consists of a WO3-coated sample as working electrode, a platinum rod as counter electrode, Ag/AgCl as reference electrode and 0.5 M LiClO4 in propylene carbonate as an electrolyte. Several voltammograms were recorded within the voltage range of + 1.5 to - 1.8 V with a scan rate of 50 mV/s. A continuous curve was observed for the films at a certain voltage sweep where a random insertion of Li+ occurred reversibly in a definite crystallographic site [WO3 (colourless) + nLi+ + ne-↔ LinWO 3 (blue)]. A dark blue colouration (%T = 20-30% in the visible region) was observed under a constant voltage of - 1.8 V whereas bleaching occurred at + 1.0 V (%T = 60-70%), which was studied simultaneously along with the voltage sweep of CV. The colouration time (Tcol) and the bleaching time (Tbl) were almost equal as revealed by the simultaneous study of the second signal (photomultiplier output coupled with the electrochemistry system) during colouration↔bleaching. Coatings of about 500 nm thickness exhibited more that 500 cycles (colouration↔bleaching). The reversibility of the cycle remained good but the intensity of the colouration decreased with the number of cycles. This was possibly due to the structural deformation of WO3 films for its long time exposure to the electrolytic solution. The cathodic current (I c) and anodic current (Ia) increased with increasing thickness. © 2003 Elsevier Science B.V. All rights reserved.