Photoetching was used to obtain a narrow gap (about 10 mu m) on a slip of indium tin oxide (ITO) which was coated on a glass plate. A high electrical field (about 10(6) V/cm) was applied between the two sides of the gap, so as to study the characteristics of ITO. Under this field, there is an intermittent current between two sides of the gap. We found that ITO decomposed, became opaque and its surface became Very rough. The atomic concentrations of In and Sn in the dark region were higher than those in the original ITO slip. The resistance of the ITO slip increased accordingly. There were some gases, such as oxygen, carbon dioxide (or nitrogen), carbon monoxide, which evolved from the surface when ITO decomposed. After quick annealing under an oxygen atmosphere, the sample became as transparent as the original ITO. These observations show that ITO decomposes under a high electric field.