The structural changes in amorphous WO3 films were investigated both on the atomic and electronic levels, and the experimental findings were interpreted using molecular orbital calculations. Electrochemical fast intercalation resulted in the splitting of a peak in the valence band region of the X-ray photoelectron spectrum. This splitting could be attributed to the formation of non-bridging oxygen. Decomposition Of WO6 units into WO4 units could also be inferred from the data. This decomposition was, however, not responsible for the split of the photoelectron peak. From the population analyses it was found that the average bond strength decreased due to the intercalation, while select W-O bonds increased in strength. It was expected that these changes in the chemical bonding character lead to localization of electrons and distortion of WO6 units, which was consistent with the theoretical interpretations of electrochromism, the intervalence charge transfer model and the small polaron absorption theory. (C) 2003 Elsevier B.V. All rights reserved.