Seeded growth reactions of the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) were performed at TEOS concentrations up to 1.2 mol/dm³. Reaction progress was followed by the in-situ measurements of pH and electric conductivity and by analyses of the final product. At a high TEOS concentration (1.2 mol/dm³), a large number of secondary particles were generated. As the TEOS concentration was reduced (0.4-0.8 mol/dm³), the secondary particles disappeared. A further reduction in the TEOS concentration (0.05-0.1 mol/dm³) led to the generation of secondary particles. From the results, the secondary particle generation could be explained by increased electrostatic repulsion forces. Further experiments performed with electrolytes confirmed this conclusion, where it was found that added electrolytes brought about the suppression of the secondary particles.