TiO2 was doped with Se(IV) ions to obtain a visible light active photocatalyst with high photocatalytic activity. In order to characterize and describe the effect of Se(IV)-doping on the electronic and structural properties of TiO2, a combination of experimental structural methods and OFT calculations were used. A series of Se(IV)-doped photocatalysts with different Se(IV) contents were prepared by an incipient wet impregnation method, followed by calcination at 623, 723 and 823 K for 1 h, 3 h and 5 h. The Se(IV)doped photocatalysts were characterized by XRD, XPS, UV-DRS, FESEM, EDX, HR-TEM and surface area (BET) measurements. A high visible light absorption band from 420 nm extending up to 650 nm in the UV-DRS was obtained. The photocatalytic activity of the Se(IV)-doped TiO2 photocatalyst was also determined by investigating the kinetics of the photocatalytic degradation of 4-nitrophenol under both UV-A and sunlight irradiation. Se(IV)-doped TiO2 enhanced the degradation rate of 4-NP. It also exhibited substantial photocatalytic activity under direct sunlight irradiation. In the computational part of the study, neutral, stoichiometric clusters Ti7O18H8 and Ti25O55H10 cut from the anatase bulk structure and two new models for the Se(IV)-doped TiO2 were developed. The DFT calculations were carried out by the hybrid B3LYP functional, by using double-zeta, LanL2DZ basis set. The experimental results combined with DFT calculations indicated that Se(IV)-doping of TiO2 enhances the visible-light photocatalytic activity by the introduction of additional electronic states originating from the Se 3p orbitals in the band gap. (C) 2012 Elsevier B.V. All rights reserved.