High-resolution electron-energy-loss spectroscopy (HREELS) showed that the dominant feature in the HREELS spectrum of the 'as-received' ITO was the loss peaks associated with the CHx group. Oxygen-plasma treatment of ITO led to the disappearance of the CHx loss peaks, and a concomitant increase in the intensity of the phonon peaks at 71 and 134 meV. The result indicates the removal of the CHx group and additional oxidation of the ITO surface by the oxygen-plasma treatment. Both processes are proposed to be responsible for the increase in work function of ITO by the oxygen-plasma treatment. (C) 2003 Elsevier Science B.V. All rights reserved.