The electronic properties and photocatalytic activity of X (N, C)/transition metal (TM = Ta, Hf, Fe)-codoped anatase TiO2 have been investigated using density functional theory. It was found that only the (N, Ta)-codoping case narrows the band gap significantly by about 0.48 eV, driven by the continuum-like p-d hybridized states above the top of valence band and d states at the bottom of conduction band. The calculated energy results suggest that codoping of Ta with N can increase the N concentration in N-doped TiO2 based on energy results. (C) 2009 Elsevier B. V. All rights reserved.