Factors for controlling sizes of silica particles formed in the hydrolysis and condensation of silicon alkoxides were examined in batch and semi-batch processes with our model previously proposed. Particle sizes in the particle formation were simulated for buffer systems to reduce time-variation in pH. Effectiveness of the buffer system to suppress time-variation in ionic strength was experimentally verified in a silicon alkoxide concentration range of 0.01-0.1 M. Comparison of experimental particle sizes with calculated ones showed that the addition of electrolytes slightly decreased surface potential of silica particles in both batch and semi-batch processes, and the surface potential values estimated for the semi-batch process were lower than that for batch process. In simulation of the number of particles formed in the processes, the particle number had strong dependences on surface potential and Debye-Htickel parameter. The simulated number of particles formed in semi-batch process was smaller than that in batch process under the same surface potential and Debye-Hfickel parameter. The combination of the low surface potential and the small number of particles revealed that the semi-batch process was suitable for producing a small number of particles, which provides enlargement of size range of silica particles formed in the method. (C) 2012 Elsevier Inc. All rights reserved.