Metal nanoparticles and carbon nanotubes have been shown to possess high electrocatalytic activity. Indium tin oxide (ITO) is a popular electrode material, but the electro-catalytic properties of its nanomaterials have not been reported. We demonstrate here for the first time facile electrocatalytic oxidation of ascorbic acid on ITO nanoparticle-modified electrodes. Compared to the conventional ITO thin film electrode, the voltammetric peak potential for ascorbic acid oxidation was lowered by 800 mV on ITO nanoparticle-modified electrodes to a potential similar to metal electrodes. The ITO nanoparticle was composed of 90% In2O3 and 10% SnO2. Since the electrocatalytic activity was also found on In2O3 nanoparticle electrodes but not on SnO2 nanoparticle electrodes, the In2O3 composition in ITO nanoparticle is mainly responsible for the high activity. In photoluminescence measurement, two intense emission peaks at 415 nm and 438 nm associated with surface oxygen vacancies were observed on the semiconductor electrodes. It was hypothesized that the oxygen vacancies could be the active sites for electrocatalytic reactions. A linear relationship between the oxidation current and ascorbic acid concentration was found in the range of 10 mu M to 5 mM, with a lower detection limit of 5 mu M and 7.9% RSD (n = 11). The high electro-catalytic activity and transmittance of In2O3 and ITO nanoparticle electrodes make them potentially very useful in opto-electronic devices and chemical/bio-sensors. (C) 2011 Elsevier B.V. All rights reserved.