MgO thin films, which are widely used in plasma display panels (PDPs), were deposited by E-beam evaporation and exposed to ambient air. The film surface changed from shiny to dull after exposure. The firing voltage of the PDP made using these air-exposed MgO thin films was increased by about 27 V. This increase is known to result from the effect of hydration, but the exact reasons are not well known. For this purpose, X-ray photoelectron spectroscopy (XPS) analysis was conducted. To dehydrate the hydrated MgO films, surface etching was conducted by an Ar ion beam. The XPS spectra showed that density of states was changed by etching the hydrated MgO films. The valence bandedge shifted to lower binding states, and intensity of the valence band spectrum also increased. Considering the secondary electron emission mechanism, these results mean that the secondary electrons in the valence band can be ejected easier with higher energy after dehydration. Therefore, electron density and energy of the electrons in the discharge area can be increased by dehydration. Consequently, the electrical properties of dehydrated MgO films can be improved. Similar behavior was observed when the film was heat-treated, but the etching process was more effective in dehydrating the films. (c) 2007 The Electrochemical Society.