Co-impregnation of vanadia and tungsta on titania produced highly active catalysts for the reduction of nitrogen oxides with ammonia but showed aging tendencies at temperatures around 600 degrees C. In order to preserve their low temperature activity after hydrothermal aging up to 650 degrees C, the catalysts were enriched with Si during the co-impregnation process using different SiO2 precursors and amounts. The optimized catalyst composition comprised ca. 4 wt% SiO2, was stable after aging at 650 degrees C, could preserve the low temperature activity and was more active than the catalysts prepared from commercially available silica-stabilized TiO2 supports. The origin of the effect of SiO2 was investigated by XRD, BET, TEM, STEM and Si-29 MAS NMR. SiO2 remained amorphous at all temperatures and 2-4 wt% SiO2 hampered the anatase crystallite growth at 600 degrees C and 650 degrees C. In contrast to WO3, SiO2 was unevenly distributed over the TiO2 particles and ca. 60-70% of Si was present as bulky amorphous SiO2 agglomerates after aging at 650 degrees C. Small and extended SiO2 polymeric entities were identified to be essential for delaying the offset of the catalyst sintering. With the addition of a small amount of SiO2 in the co-impregnation process, the low temperature activity of these catalysts could be preserved up to an aging temperature of 650 degrees C.