Four N-I co-doped TiO2 catalysts having Ti:N/I molar ratios of 1:1, 1:3, 1:5 and 1:10 were prepared via a sol-gel method using NH4I as N-I dopant precursor. A pure TiO2 (undoped) sample was also prepared by the same method for comparison. The catalysts were evaluated for the simultaneous photocatalytic reduction of Cr(VI) and oxidation of benzoic acid (BA). TiO2 anatase phase was formed for all N-I co-doped catalysts as shown by XRD. UV-vis diffuse reflectance spectra showed that N-I co-doping resulted in increased absorption at visible wavelengths and a decrease of the band gap energy (E-g). The smallest E-g value of 2.34 eV was observed for the 1:5 Ti:N/I molar ratio. The structure and photodynamics of the TiO2 catalysts was investigated in detail by Electron Paramagnetic Resonance (EPR) spectroscopy. The EPR data showed: [i] formation of non-photoactive NO centers and photoinduced N-b(center dot) paramagnetic species as a result of N doping, [ii] photoinduced Ti3+ surface ions, and [iii] formation of surfacial oxygen O-2(center dot-). radical ions and trapped holes TiO4+-O center dot-. The N-b(center dot) species act upon the narrowing of the band gap and the production of photogenerated electrons, i.e. Ti3+ surface ions. These Ti3+ surface ions have a key role, capturing gas O-2 and supporting both reduction and oxidation process. Cr(VI) reduction by the N-I co-doped catalysts followed the trend: TNI5 > TNI10 > TNI1 > TNI3 > TiO2[undoped] which correlates with the concentration of N-b(center dot) species formed and the narrowing of E-g values. Oxidation of benzoic acid (BA) followed a more complex trend as follows: TNI1 > TNI3 > TNI10 > TNI5 which is the reverse of the trend for trapped holes TiO4+-O center dot-. Finally progressive microwave EPR saturation experiments show that N-b(center dot) species are located deeper in the TiO2 lattice in particles with narrower E-g values. (C) 2013 Elsevier B.V. All rights reserved.