Surface states at the nanostructured TiO2 (anatase)/aqueous electrolyte interface have been investigated using spectroelectrochemical methods. It is found that electrons trapped in these states have an absorption spectrum that differs significantly from that of conduction band electrons. At 400 nm the extinction coefficient of an electron trapped in a surface state is determined to be 1900 M-1 cm(-1), more than 3 times higher than that of a conduction band electron at the same wavelength. Surface states are located about 0.5 eV below the conduction band edge. Their density is determined to be 3 x 10(12) cm(-2) (microscopic area).