Seeded reactions of the hydrolysis and condensation of tetraethyl orthosilicate were carried out to study the influence of ammonia concentration on particle size distributions. Aggregation of seed particles was observed at a high ammonia concentration (1.0 kmol/m(3)), whereas a large number of new particles were formed at a low ammonia concentration (0.25 kmol/m(3)). An intermediate ammonia concentration (0.5 kmol/m(3)) was found to be suitable for particle growth that was scarcely accompanied with aggregation and new particle formation. Estimation of ionic strength from pH values during the reactions suggested that an increase in the ionic strength with an increase in ammonia concentration suppressed the new particle formation and lead to the particle aggregation. Seeded growth reactions in which KCl or LiCl was added demonstrated that new particle formation was highly suppressed by the addition of the electrolytes.