The structures of V2O5 catalysts for the selective catalytic reduction (SCR) of NO by NH3 at low temperatures are investigated using in situ V K-edge X-ray absorption near-edge structure measurements. The V2O5 catalyst supported on TiO2-SiO2-MoO3 mixed oxide (V/TSM) showed a significantly steep increase in the main-edge energy at the beginning of the reaction. The results indicate that the excellent NH3-SCR catalytic activity of V/TSM can be ascribed to the remarkable V4+-to-V5+ reoxidation capability and easier V5+-to-V4+ reducibility. This improved redox reactivity is induced by the interaction of the highly dispersed polymeric vanadate with TSM solid solution and by the appropriate surface geometry of vanadium species, which enabled contact with NH3 and O-2 in the gas phase. The results reported herein should enable the development of improved catalysts for NH3-SCR.