In this work, various Al2O3/SiO2/TiO2 nanocomposites prepared from organic precursors through a sol-gel method and loaded with vanadium were studied for the selective catalytic reduction of NOx with ammonia as a reductant. The coexistence of Al and Si as heteroatoms in the titania matrix resulted in lower levels of activity than titania catalysts due to the low crystallinity of titania in the polyoxide supports. Catalytic experiments indicate that the level of NOx conversion and N-2 selectivity for the Al-rich or Si-rich catalysts are comparable under identical operating conditions. However, the NOx conversion over the SiO2-rich polyoxides passes through a maximum for composites with atomic composition Ti:Si:Al=30:5:1. This behavior is strongly related with the BET surface area of each catalyst. The NOx conversion and N-2 selectivity over the Al-rich catalysts follows very closely their BET surface area as well. The Bronsted acidity of the support, which is introduced by the addition of Si and the Al dopants, seems to play a secondary role in the catalyst performance. The addition of Si and Al as heteroatoms decreases the temperature window for optimum performance in comparison with earlier studies for SCR reactions. FT-IR, XRD, BET and TEM were used for the characterization of our catalysts.