Alternate layer-by-layer assembly of colloidal SiO2 particles with polycations has been investigated by quartz crystal microbalance (QCM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). QCM measurement confirmed the high regularity and reproducibility of the assembling process that depends on particle concentration, particle size, and ionic strength. The individual adsorption step was completed within 15 s. The thickness of adsorbed layers increased with increasing SiO2 concentrations at the three particle sizes used (45, 25, and 78 nm in diameter), unlike the case for other polyion assemblies. It also increased with increasing ionic strength of aqueous SiO2 dispersions. According to SEM observation, the assembled film possessed surprisingly flat surfaces at optimized ionic strengths. AFM observation revealed that SiO2 particles were not closely packed. The neutralization ratio of SiO2 and PDDA was estimated by turbidity measurement. Comparison of turbidity and QCM data indicated t...