A series of experiments was conducted to reveal the impact on the catalyst and NH3-SCR activity of an industrial deNO(x) catalyst from Haldor Topsee A/S (V2O5-WO3/TiO2) by exposure to H-2 and H-2/O-2. Treatment of the SCR catalyst was done by exposure for 60min with three different concentrations of H-2 (0-2%) in an 8% O-2/N-2 mixture, where the SCR activity was measured before and after the hydrogen treatment. The results show that the activity of the SCR catalyst is only negligibly affected during exposure to the H-2/O-2 gas and in all cases it returned reversibly to the initial NOx conversion rate after reexposure to the standard NH3-SCR gas. In situ electron paramagnetic resonance (EPR) and UV-vis spectroscopy at 250 degrees C suggest that a significant part of V4+ present during SCR conditions was oxidized to V5+ during exposure to 2% H-2 + 8% O-2. However, the distribution of vanadium in oxidation states V4+ and V5+ returned to the initial redox equilibrium in less than an hour after the hydrogen-oxygen treatment, emphasizing the reversibility of the v(4+)-V5+ system. (C) 2011 Elsevier B.V. All rights reserved.