N-doped TiO2 nanocrystals were successfully synthesized by a controlled sol-gel reaction at pH 9 followed by calcinations at 400 degrees C. Triethylamine was used to modulate the N content of the samples. The photocatalytic activity was tested using a solar irradiation source with regard to two different pollutants: NOx (in the gas phase) and methylene blue (MB), deposited onto the oxide film. In the case of MB degradation the disappearance of the (MB) blue colour was measured directly onto the solid, by means of UV-vis-NIR diffuse reflectance spectroscopy. The final mineralization of the organic molecule was assessed by COD determinations. All N-doped samples produce a better removal of both pollutants than the undoped and commercial samples. By coupling structural (by X-ray synchrotron radiation) and morphological (HRTEM, BET, granulometry) characterizations with spectroscopic analyses (EPR and XPS), the presence of paramagnetic centers, defects, edge dislocations and surface states is appreciated and found to affect the material photocatalytic behaviour. (C) 2010 Elsevier B.V. All rights reserved.