The changes in the structure and optical properties of indium tin oxide (ITO) thin films that were subjected to water electrolysis using an alternating current power source were investigated. Xray diffraction indicated that the film was reduced to metallic indium after the treatment. Scanning electron microscopy revealed that the film became porous after the treatment. These structural changes had led to a dramatic degradation in the optical properties of ITO. The electrochemical mechanism responsible for the changes was discussed.