In order to clarify the basic reason why Ce doping call dramatically decrease the leakage current in Ba0.5Sr0.5TiO3 (BST) as reported in our previous work (Wang et at 2005 J. Phys. D: Appl. phys. 38 2253), we have employed x-ray photoelectron spectroscopy (XPS) and the optical transmittance technique to Study the electronic structure of undoped and 1.0 at% Ce-doped BST (CeBST) films fabricated by Pulsed laser deposition. XPS results show that Ce doping has a strong influence on the valence band and core levels of BST films, and that the Fermi level is lowered by about 0.35 eV by Ce doping. Optical transmittance measurements demonstrate that the energy,gap is expanded with Ce doping. These Ce-doping effects can induce an increase in the barrier height for the thermionic emission and eventually reduce leakage current in CeBST thin films.