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Lattice location of B in TiO 2 is tuned to determine its effect on the photocatalytic activity of Co-B codoped TiO 2 . Sol–gel method was used to synthesize the samples. The concentrations of Co and B were first optimized by maximizing the photocatalytic activity for the monodoped (Co or B)-TiO 2 . In addition to the DFT calculations for discovering new energetic levels introduced in TiO 2 by codoping, various characterization techniques were used to determine the dopant lattice sites in TiO 2 and interactions between them; and also determining their consequences on electronic, morphological, structural, and optical properties. At low concentration of B-doping (1 at.%), B occupies the interstitial site (B int ), but as the concentration increases (2 at.% and 3 at.%) B also occupies substitutional O position (B sub ) in addition to B int to form TiO 2 containing B int+sub . Both these B-doped TiO 2 showed improved photocatalytic activity attributed to effective charge separation obtained for TiO 2 –B int due to the formation of shallow energy level while higher visible light absorption is achieved with TiO 2 –B int+sub owing to the presence of two deep energy levels in the band gap as confirmed by DFT calculations. In the case of Co doping, the band gap of TiO 2 is reduced but the recombination rates are always high and are caused by the formation of Co states in the band gap. For Co monodoped TiO 2 , the photocatalytic activity is low for all the concentrations considered, except for very low concentration of Co (0.1 at.%). Two opposite effects were observed when small amount of Co (0.1 at.%) was codoped with B int or B int+sub . In particular, the photocatalytic degradation rate of organic aqueous pollutants ( p -nitrophenol and rhodamine B dye) reduces for TiO 2 –Co–B int whereas it is enhanced remarkably for TiO 2 –Co–B int+sub as compared to (Co or B) monodoped (∼2.1 times) and undoped (∼7.8 times) TiO 2 . Higher photocatalytic activity observed in Co-doped TiO 2 –B int+sub is discussed in terms of the interactions of Co with B at two different lattice positions in TiO 2 and the synergistic effect created by higher visible light absorption and the improved charge separation. [ABSTRACT FROM AUTHOR]