In the present study, microstructural transitions of silica suspensions were examined by rheological measurements under either steady simple shear or oscillatory flow. First, monodisperse silica particles were prepared by the so-called modified Stober method and were stabilized sterically in organic medium. Silane coupling agent, such as 3(trimethoxysilyl)propyl methacrylate or N-[3-(trimethoxysilyl)propyl] ethylenediamine (aminosilane coupling agent) was coated onto the particle surface to induce steric stabilization at various volume fractions (phi) up to phi = 0.45. To ensure 'hard' sphere suspension, the silica particles were dispersed in a refractive-index matching solvent, tetrahydrofurfuryl alcohol, in which the van der Waals dispersion forces diminished. Second, charge stabilized silica particles were prepared in aqueous medium. In this case, ionic strength was adjusted by KCl concentration after surface modification with amino silane coupling agent. The charge stabilized suspension showed stable shear-thinning behaviour, which was well contrasted by the hard sphere suspension. The latter exhibited negligible electrostatic repulsion and only short-range interactions. Finally, microstructural transition of aqueous silica suspensions from liquid- to solid-like structure was examined in terms of particle volume fraction and salt concentration. (C) 2001 Elsevier Science BN. All rights reserved.