The use of modern spectroscopic techniques in the study of the formation of aqueous silica gels is described. The oligomerization process of monomeric silicic acid was studied by silicon-29 NMR spectroscopy; at high pH values cyclic trimeric silicate species were favored compared to the linear structure. Aggregation of primary silica particles of molecular size (<1 nm) was studied by analysis of small-angle x-ray scattering patterns. All systems studies (pH 4.0) indicate reaction-limited aggregation. Polyvalent cations influence the rate of aggregate formation in a negative way: aluminum at low concentrations (1 mol%) significantly inhibits aggregation. A new model for the aging process that proposes that monomeric silicic acid is transported (via solution) from the periphery of the aggregate into the core of the solution is given.