TiO2 nanopowder with a large surface area and high crystallinity was synthesized by a thermal decomposition process. The physicochemical properties of the prepared powders were examined by X-ray diffraction, transmission electron microscopy and nitrogen adsorption-desorption isotherms. The nanocrystallites of the prepared powers were considerably smaller than those of the commercial photocatalyst (Degussa, P25), and the particles had a dense polyhedral structure. In addition, the particles had a mainly disordered mesoporous structure with a pore volume that varied according to the pore size in the range of 2-20 nm. The photocatalytic activity of the prepared photocatalyst was obviously higher than that of P25 on the photodegradation of gaseous nitrogen oxides under UV254+185 nm lamp irradiation. Above 40% relative humidity, the NOx removal efficiency of the prepared photocatalyst was 10% higher than that of P25. Furthermore, a suitable relative humidity and longer residence time were found to enhance the photocatalytic oxidation of gaseous nitrogen oxides by UV254+185 nm lamp irradiation and TiO2 nanoparticles. (C) 2010 Elsevier B.V. All rights reserved.