The semiconducting properties of anodic grown TiO2-nanotubes were analyzed by means of the potential dependence of the interfacial capacity under different electrolyte and illumination conditions. The experimental results offer evidence for a duplex oxide film conformed by the bottom and wall of pores with different density of donors and concentration of surface states. The surface extension offered by the walls of oxide tubes is manifested in surface related effects such as the shift of the flat band potential and trapping of photoelectrons. (c) 2006 Elsevier Ltd. All rights reserved.