The polymerization of silicic acid through the stages of oligomers, nucleation, and growth of silica particles was investigated in 0.1 M silica solution at pH 1.75 and 25ยฐC. The nuclei and larger polymer particles were separated from monomer and oligomers by extraction with tetrahydrofuran from samples of solution saturated with sodium chloride. The nuclei and larger particles dissolve in molybdic acid solution to form the yellow silicomolybdate ion as a first-order reaction for which rate constants were calculated. Based on data available in the literature, particle size was estimated from the rate constant. At an early stage, the particle size was confirmed by ultrafiltration. The nuclei appear in about 2 hr and are about 1 nm in diameter. These grow first at the expense of monomer and oligomers and after 40 hr mainly by Ostwald ripening, reaching about 4 nm in size in 20 days. During this period, the concentration of monomer, Si(OH)4, decreases as particle size increases, According to the Ostwald-Freundlich equation, the interfacial energy appears to be about 100 erg cm -2 which is double that reported for particles formed above pH 7.