We employed DFT calculations to study the geometric and electronic structures of N- and NH-doped anatase TiO2 systems. With N dopant concentrations of 2.1% and 1.0%, the substitutional NH-doping resulted in bandgap narrowings of 0.12 and 0.13 eV, respectively, very close to that resulted from substitutional N- doping (0.12 and 0.11 eV). For interstitial N- and NH-doping with a nitrogen concentration of 2.1%, only NH-doping resulted in a bandgap narrowing of 0.07 eV. For both substitutional and interstitial NH-doping con. gurations, the crystalline structure adjacent to the NH dopants was heavily distorted, which might enhance the photocatalytic efficiency. (c) 2008 Elsevier B.V. All rights reserved.