The fractal character of silica aerogels made under controlled conditions has been investigated by small angle neutron scattering. The conditions involve the selection of the catalysts and the concentration. By a strong dilution, material can be prepared which is fractal over 2 orders of magnitude in length scale. The catalyst allows modification of the spatial arrangement of the elementary particles. The fractal dimension is close to 1.8 under basic conditions and within the range 2.2 to 2.4 under acidic and neutral conditions. © 1990.