Amorphous WO3 (a-WO3) films have been produced by electrodeposition from a sodium tungstate-based aqueous electrolyte. Their coloration under the action of cathode current in IN H2SO4 is accompanied by a reversible shift of similar to0.42 eV in the fundamental absorption edge of the oxide towards higher quantum energies. The shift of the edge is proportional to the change in the potential of the WO3 electrode being colored and is caused by the sequential filling, by injected electrons, of levels in the energy spectrum of electronic states formed by the unoccupied d-orbitals of W6+ centers. The optical characteristics of the W5+ centers which are formed in this case (color centers of electrochromic material) depend on whether a particular tungsten atom has a double bond to the oxygen atom (W=O type bond). At the initial stage of coloration, injected electrons are captured mainly by the W6+ centers that have no such bonds. Then, W6+ centers with W=O bonds, which have a higher position of the unoccupied d-orbitals on the energy scale, are also filled; this is accompanied by the appearance of an extra absorption band with maximum at similar to2 eV in the optical spectrum of films. (C) 2004 Elsevier Ltd. All rights reserved.