The effect of oxygen plasma treatment on the surface properties of indium-tin oxide (ITO) anodes and the changes in surface properties of treated ITO anodes with aging time were investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements. It was observed that oxygen plasma treatment increased the oxygen concentration, decreased the carbon concentration and raised the ITO work function, and thereby improved the surface properties of ITO. With the increase of aging time, however, the improved ITO surface properties tended to decay and the ITO work function decreased. In addition, the aging effect of treated ITO anodes on the performance of organic light-emitting devices (OLEDs) was studied with respect to the driving voltage, electroluminescent luminance and efficiency. Experimental results reveal that the ITO anodes aged for various times result in significant differences in electrical and optical characteristics, which become worse as the aging time increases. (c) 2004 Elsevier B.V All rights reserved.