We have attempted to modulate the band gap of anatase-titania using hybrid density functional theory (DFT) calculations to examine cation codoping at two Ti sites by Al with either Nb or Ta, and by Cu with either Mo or W to achieve significant gap narrowing via creation of shallow gap states, without creating recombination centres. Cu- and Mo/W-doped systems exhibit larger gap-narrowing than those of Al- and Nb/Ta-codoped TiO2 owing to strong d-d repulsion in the former two cases. At the same time, codoping serves to decrease transparent conducting behavior relative to monodoping. Upon increasing the doping concentration, the gap-narrowing mechanisms and transparent conducting characteristics do not change. (C) 2011 Elsevier B.V. All rights reserved.