The photoreactivity of titania (TiO2) nanoclusters with varying levels of N or Cr-doping, or (Cr, N)-codoping, was systematically investigated using photodegradation of methyl orange in aqueous suspensions. The shifting of the TiO2 absorption edge into the visible spectral region that is primarily attributable to band gap narrowing was found to be a reliable metric for estimating the photoreactivity of the doped nanoclusters. Compared to the weak response with undoped and N-doped TiO2, Cr-doping and (Cr, N)-codoping were found to significantly enhance photodegradation of methyl orange under visible light. The initial reaction rates increase from about 0 to above 1.6 x 10(-2) min(-1) when the doping concentration of Cr in TiO2 increases from 0 to 5%. In stark contrast. under UV irradiation, doping is not only ineffective but also detrimental to the photoreactivity, and all doping including N or Cr only and (Cr, N)-codoping were found to reduce photoreactivity. (C) 2011 Elsevier B.V. All rights reserved.