The removal efficiency of nitrogen oxides (NOx) at low temperature depends on the titania support. This phenomenon Was found in laboratory scale experiments using various catalysts which have different properties of titania Supports. In order to identify the factors that affect the selective catalytic reduction (SCR) activity at low temperature, the physicochemical properties of titania support such as surface area, particle size, crystal phase. crystallinity, impurities and the vanadium Compound structures of catalysts were analyzed and compared with NOx removal efficiency at low temperature. Although those properties could be important in catalytic activity, no direct relationship between those properties and NOx removal efficiencies at low temperature could be revealed. Because SCR depends on a redox reaction, electron transfer can participate in the reaction. Using X-ray photoelectron spectroscopy (XPS) method to analyze the electronic state of the catalysts, it was found that V/TiO2 catalysts contain not only Ti4+ but also Ti3+ and Ti2+ called non-stoichiometric Tiy+ (y <= 3), while the vanadium-free titania support consists of only Ti4+. Moreover V has various valence states such as V5+ and Vx+ (x <= 4),which contains V4+ V3+, etc. The SCR catalytic activity increases with increasing the amount of non-stoichiometric species per Unit Volume. Those non-stoichiometric species are more active than the stoichiometric species such as V5+ and Ti4+ due to their metastable properties. Therefore, the main factor affecting the low temperature SCR is the presence of non-stoichiometric species of catalysts and their amount per unit volume. (C) 2009 Elsevier B.V. All rights reserved.