An X-ray photoelectron spectroscopy (XPS) study has been performed for (0 0 1) BaB2O4. The crystal surface has been polished mechanicaIly and cleaned by chemical etching. In XPS observation, depth profiling has been produced by sputtering with Ar+ 3 keV ions. Photoelectron binding energies of original element core levels and valence band have been measured as a function of sputtering time. The persistence of binding energies of barium and boron core levels and valence band structure has been found. For O 1 s core level. the formation of new spectral components with lower binding energies has been revealed. (C) 2003 Elsevier B.V. All rights reserved.