The electrochemical behavior of tin-doped indium oxide (ITO) on SiO2 in 0.3 M HCl was studied using voltammetric scanning method. The result showed that an obvious reduction current peak occurred during the first cathodic potential scanning. The reduction reaction became less active after annealing ITO at 500 C for 1 h. The result was attributed to the replenishment of oxygen-deficient site, which acts as current carrier, by the annealing treatment. Many spherical In-Sn particles with sizes about 100-500 nm were formed on the ITO surface adjacent to the grain boundary when the reduction current took place. The In-Sn particles initiated preferentially on the ITO surface near grain boundaries attending the dissolution of ITO grain boundary. In the scanning period, after completion of the reduction current peak, reduction of hydrogen ions occurred in more negative potential region. A schematic illustration for the formation mechanism of the In-Sn reduction particle was postulated based on the metallographical and electrochemical results in this work. (C) 2003 Elsevier Ltd. All rights reserved.