Three types of aqueous sodium silicate solutions characterized by molar SiO2:Na2O ratios of 2.2, 3.3, and 3.9 were investigated by dynamic light scattering and viscosity measurements. The solutions were prepared by diluting concentrated commercial products to SiO2 content between 0.5 and 15 wt%. Dynamic light scattering (DLS) was used for the investigation of size distributions of colloids immediately after dilution. At least three size classes of colloidal particles were detected, which exist in parallel in the solutions. The respective radii are 0.4 to 0.6 nm, 2.5 to 13 nm, and 75 to 85 nm. The larger colloids dominate the mass distribution, whereas the smaller colloids control the surface area of the colloidal fraction. Material properties such as pH, viscosity, refractive index, and density were measured as a function of dilution. The evaluation of the viscosity data indicated that the colloids have a lower density than dense amorphous silica. Aging effects could be suppressed by measuring within 1 h after dilution. (C) 2003 Elsevier Inc. All rights reserved.