The particle formation mechanism during hydrolysis and condensation of tetraethyl orthosilicate (TEOS) was studied with in-situ measurements of particle size distribution, electric surface potential and electric conductivity in the absence and in the presence of electrolytes, KCl and LiCl. Experiments were performed at a TEOS concentration of 0.4 mol/dm³, a water concentration of 11 mol/dm³ and an ammonium concentration of 1 mol/dm³. The addition of KCl has a greater effect on the particle sizes than that of LiCl. The average particle size attained 1 μm at a KCl concentration of 4 mmol/dm³ maintaining high monodispersity in particle sizes. Measurements of surface potential and electric conductivity show that the increase in particle size by the addition of the electrolytes is caused by both a reduction in the surface potential and an increase in the ionic strength. The effect of the electrolytes was also examined in seeded growth experiments. Addition of KCl to the system suppresses the generation of new particles and enables the growth of seed particles by as much as fifty-fold in volume. Our experimental results show that electrostatic interaction between the particles is the dominant factor in controlling particle size distributions.