The influence of solids loading and the electrical double layer on the rheological behavior of concentrated silica nanosols was investigated. In this study, silica suspensions were characterized by viscosity, electrokinetic sonic amplitude measurements, and by theoretical considerations. Despite a high electrokinetic potential at pH 8, fumed silica not only exhibits rheological behavior normally indicative of an unstable suspension, but the rheology does not have the expected DLVO dependence on ionic strength. Normalization of viscosity to an effective volume fraction that incorporates the influence of the electrical double layer, leads to partially superimposable curves. The positive correlation between zeta potential and viscosity indicates that classical DLVO calculations alone are insufficient to predict the rheological behavior of concentrated silica nanosols, without first taking into account the effects of particle crowding and the repulsive interactions on suspension structure. The relatively low magnitude of van der Waals attractive forces between silica particles in water also plays an important role at high electrolyte concentrations. A comparison is made between silica nanosols and silica microspheres, and also between nano-silica and nano-alumina. (C) 2004 Published by Elsevier B.V.