High-surface-area alumina–silica mixed oxide (Al2O3:SiO2) nanocatalysts have been prepared by a hybrid sol–gel method using boehmite (synthesized from aluminum nitrate) as the source of alumina and tetraethyl orthosilicate as the source of silica. The gels, after calcination at 400°C, result in mixed oxides with specific surface areas of 287 m2/g (Al2O3:SiO2=3:1) and 262 m2/g (Al2O3:SiO2=3:4). Further heating to 600°C produces materials with specific surface areas of 237 and 205 m2/g, respectively. The larger specific surface areas characteristic of the 3Al2O3:SiO2 samples are attributed, via transmission electron micrograph investigations, to the presence of ∼10 nm size, needle-like particles having an aspect ratio of 1:50. Further addition of silica leads to the formation of larger needles of 20–75 nm size. Calcination at 600°C induced an approximately 5% decrease in the total pore volume for the 3Al2O3:SiO2 sample. In contrast, the material with Al2O3:SiO2=3:4 showed an approximately 12% increase in pore volume when heated at 600°C. The pore-size distribution was in the range 1–3.5 nm with rmax at ∼2 and ∼2.5 nm at 600° and 800°C, respectively. Adsorption isotherms and pore-size distribution analyses are discussed in some detail for the aluminosilicates at different calcination temperatures. This discussion is supported by structural information determined from FTIR and 27Al MAS NMR studies. Relatively high acidity values (0.234 mmol/g for Al2O3: SiO2=3:4) are observed for silica-rich compositions consistent with their application as efficient acid catalysts.