An unoccupied density of states (DOS) evolution under oxidation of ultrathin Mg, Zn and Cd layers on SrTiO3(1 0 0)-(2 x 2) substrates are investigated. The metal deposition and subsequent oxidation were studied in situ under UHV conditions by means of LEED, AES, TCS and work function measurements. An appearance of metal- and oxygen-derived bands of the unoccupied DOS was traced in the TC spectra. Metal oxidation was followed by the hybridization of metal and oxygen orbitals and by the appearance of the mainly O 2p oxygen-derived bands, which are characterized by a set of DOS maxima, located at the following energies (above the Fermi level): 8.0, 11.5, 14.3, 15.0 and 19.0 eV for MgO; 6.0, 8.1, 16.8 and 20.5 eV for ZnO and 7.5, 10.0 and 12.7 eV for CdO. The results obtained are discussed in comparison with previously published experimental and theoretical data on unoccupied DOS in metal oxides. (C) 2001 Elsevier Science B.V. All rights reserved.