Small angle neutron scattering results obtained on several series of silica aerogels are presented. It is shown that basic catalysis can produce materials that are fractal over at least two orders of magnitude in length. These have bulk densities lower than ∼ 0.02 g/cm3, corresponding to porosities of 99% or larger. In this case, the constituent particles, whose gyration radii are larger than 10 Å, have a fractal surface that cannot be smoothened by oxidation. Densification at elevated temperatures mostly increases the particle size. The fractal structure is destroyed when the particle dimensions equal the correlation length at the gel-glass transformation. © 1988.