The formation of colloidal silica particles and the dynamics of the nanostructure of the particles are investigated by small-angle X-ray scattering (SAXS) technique. Solute concentrations of 0.5 M tetraethylorthosilicate (TEOS), 1.1 or 2.2 M water (H2O), and 0.04 or 0.1 M ammonia base (NH3) in ethanol were used to obtain reaction conditions as close to those of the Stober method as possible and to have reaction kinetics that were slow enough to probe the changes in the nanostructure of the growing particles and to obtain good statistics from the SAXS measurements. We measured the changes in the radius of gyration and the fractal dimension as a function of time during growth. Remarkably, we find that, after an induction period, the first particles to appear in the solution have a radius of gyration of approximately 10 nm and are mass fractals characterized by their polymeric, open structure. This stage is followed by an intraparticle densification process and smoothing of the interface, leading to the usual compact nonfractal, stable structures. The growth models proposed so far cannot account for the observed continuous changes of stages during the formation and growth of the particles. Copyright 1997 Academic Press. Copyright 1997Academic Press