The key aspects related to surface states present on an anodically formed TiO2 nanotube array are here discussed. Impedance measurements performed on similar layers, in solutions at different pH, evidenced quite different behaviors and conducting properties. The comparison between the surface state capacitances at three different pH values showed that surface states have to be correlated with hydroxyl groups at the surface of TiO2. They are characterized by a broad distribution in energy in the band gap. Under polarization in alkaline solution, charge transfer then mainly occurs through the surface states. The situation is quite different in acidic solutions in which a low density of surface states was measured and in which proton insertion and negative charge storage in the tubular layer is responsible for the high doping level of the tube walls and band bending in a narrow space charge zone in the walls. In neutral solutions at pH close to the zero-charge pH, only the bottom of the tubes is active, and a localized state attributed to the adsorption of undissociated water molecules is observed.