Suggested mechanisms for NH 3 À SCR on V 2 O 5 /TiO 2 based catalysts contain both oxidized and reduced vanadium species. The degree of reduction and the importance of the redox properties of the catalyst are however still widely discussed. Here we show the presence of significant amounts of oxidovanadium(IV), VO 2 + using electron paramagnetic resonance (EPR) spectroscopy of V 2 O 5 /TiO 2 and V 2 O 5 /WO 3 /TiO 2 under operando selective catalytic reduction (SCR) of NO with NH 3 in a fixed bed plug flow reactor operating at high space velocity: 420,000 h À 1. The V(IV) content at the completely oxidized and reduced states is also analyzed. It is found that a significant part of the vanadium is reduced to the EPR active species, VO 2 + at operando SCR conditions and even more when O 2 is removed from the SCR gas.