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 °C. In order to preserve their low temperature activity after hydrothermal aging up to 650 °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 °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 ²⁹Si MAS NMR. SiO₂ remained amorphous at all temperatures and 2–4 wt% SiO₂ hampered the anatase crystallite growth at 600 °C and 650 °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 °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 °C.