The photoelectrochemical behavior of nanostructured TiO2 (consisting mainly of anatase) films, with thicknesses ranging from ca. 0.6 to 45 mu m, was examined under the band gap UV illumination. The shape of the spectral photoresponses exhibiting a maximum at 300 nm, irrespective the film thickness, and the excellent photocurrent-voltage characteristics of the thickest films are inconsistent with the previously proposed models aimed at describing electron transport in such nanoporous semiconductor networks. It was found that even electrophoretically deposited nanoparticulate films, simply dried at room temperature, behaved essentially as the high-temperature-annealed films. An explanation of the particular behavior of the nanostructured anatase films, in terms of the self-doping occurring at the initial stages of the photocurrent flow, is proposed. This initial film charging causes an insulator-metal (Mott) transition in a donor band of anatase, accompanied by a sharp rise in conductivity of the nanoparticles. Such a self-doping appears as a special feature of nanostructured semiconductor films filled with the electrolyte and characterized by large surface-area-to-volume ratios. This offers a convenient way of compensating the excess charge within the semiconductor by the adjustment of the ion concentration in the Helmholtz layer.