The rate of the selective catalytic reduction of NO with N-containing reducing agents may be considerably enhanced by converting part of the NO into NO2. The reaction using an equimolar mixture of NO and NO2 is known as "fast SCR reaction" and the rate enhancement is most pronounced at low temperatures (T < 300degreesC). In the present study the possible role of NO2 on catalysts based on TiO2-WO3-V2O5 was investigated. Transient experiments in a test reactor and in situ Raman spectroscopy investigations were performed to study the reoxidation of the vanadium species in the catalyst. The experiments suggest that the V+4 species formed during the reduction of NO with ammonia are reoxidized faster by NO2 than by oxygen, resulting in an increased reaction rate of the fast SCR reaction.