The electrochemical reduction of indium tin oxide (ITO) on glass is systematically studied. It is revealed that the light absorbance and electric resistance of ITO increases upon reduction. Scanning electron microscopy (SEM) images show that the integrate ITO films dissolve and form particles upon applying negative potentials. The particles consist of metallic In and Sn, as characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The reduction of ITO strongly depends on the electrolyte conditions, mainly pH and anions. The onset potential is found to shift negative as the pH of the electrolyte increases. NO3- ions are found to significantly inhibit the reduction of ITO, shifting the reduction potential negative by ca. 500 mV as compared with SO42-, Cl- and Br-. It can also serve as inhibitor by adding very low concentration to the Cl--dominant electrolyte. Moreover, it is found that the electrochemically reduced ITO show excellent non-linear optical performance, with transmittance tuneable by reduction potential and time. This suggests a promising useful application of the electrochemical reduction of ITO. (C) 2015 Elsevier Ltd. All rights reserved.