Doping of nanocrystalline TiO2 powders with sizes ranging from 9.5 nm up to 19.1 nnn was carried out with thiourea (TU) to introduce the C, N and S-species into the TiO2. The crystal size was determined by X-ray diffraction. Doped-TiO2 particles were colored while the undoped TiO2 was white. The edge of the absorption band edge was analyzed to describe the TiO2 band gap according to the indirect transitions theory of semiconductors. In the former analysis the energy distribution of the doped centers is discussed in terms of the Urbach's tail theory (K = K(0)exp(sigma((h nu - E-g)/kT))), being K the absorbance in the Kubelka-Munk equation. Evidence is presented for thiourea doped-TiO2 leading to Urbach's tail associated with the formation of states in the band-gap rather than a narrowing of the band gap. The photocatalytic activity of the doped-TiO2 was tested by the reduction of tetra-nitromethane to nitroform and by the oxidation of I- to I-3(-). The rate of formation of nitroform (NF) produced under 460 nm light increased for the smaller TiO2 nanoparticles. This suggests the formation of localized centers under 460 nm light. But under 366 nm light, the highest rate of NF formation was observed for samples with the biggest nanocrystal size. Under 366 nm light, the smaller particles show a low rate of nitroform formation due to a more favorable band-gap electron-hole recombination. Lipid phosphatidylethanolcholine (PE) under 460 nm visible light in the presence of doped-TiO2 led to the formation of conjugated of double bonds in PE. This implies the formation of peroxy radicals due to the TiO2 e(cb)(-) localized electronic states under visible light irradiation. (C) 2009 Elsevier B.V. All rights reserved.