It was confirmed that the divalent cations, Zn2+ and Cu 2+ were electrochemically inserted into amorphous WO3 films prepared by r.f.-magnetron sputtering method. Coloration of the films was also observed, and the films were, however, bleached gradually. The bleaching mechanism was discussed based on the ab-initio molecular orbital calculations, in which Li+ or Zn2+ ions were introduced into the WO 3 frameworks. In the case of Zn insertion, electrons were trapped in a non-bonding Zn 4s orbital just below the conduction band. It was finally concluded that in the films with distorted WO3 frameworks, Zn 4s orbital was mingled in the conduction band and electrons were free to be transferred between W6+ and W5+. As the structural relaxation progressed, electrons were gradually trapped in Zn 4s orbital, and the films were finally bleached.