The BaO thin film was grown on the Si(0 0 1) surface and its surface properties were investigated using metastable impact electron spectroscopy (MIES) and coaxial impact collision ion scattering spectroscopy (CAICISS). The BaO surface reduced by annealing at 600 degreesC in ultrahigh vacuum is covered with a metallic Ba overlayer and exhibits the work function as small as 0.5 eV. The semiconducting property recovers upon oxygenation of this surface as revealed from the MIES spectra. It is found that film quality is degraded considerably after several reduction/oxygenation cycles. Probably, this is because a template BaSi2 structure is broken gradually as a consequence of the solid-state reaction between BaO and BaSi2, which is induced by the migration of chemisorbed oxygen to the interface. (C) 2002 Elsevier Science B.V. All rights reserved.