The passivation of sol-gel TiO2 by ultra-thin layers of Al-oxide has been investigated using transient and spectral photovoltage (PV) techniques. The ultra-thin layers of Al-oxide were prepared by the ion-layer gas reaction (ILGAR) technique and modified by thermal treatments in air, vacuum or Ar/H2S atmosphere. The samples where characterized by elastic recoil detection analysis (ERDA), X-ray photoelectron spectroscopy (XPS), and contact potential difference (CPD) technique. Without an Al-oxide surface layer, electronic states in the forbidden gap of TiO2 are formed during thermal treatments in vacuum and Ar/H2S. The trap density is strongly reduced at the TiO2/Al-oxide interface. The formation of electronic defects is prevented by a closed ultra-thin layer of Al-oxide. (C) 2004 Elsevier B.V. All rights reserved.