The theological behavior of concentrated aqueous suspensions of nearly monodisperse submicron, spherical silica particles was studied in Couette and vane geometries. End corrections and wall depletion effects were found to be important. The apparent yield stress and shear viscosity were investigated in the light of interactions between the charged silica particles. The effects of pH as well as the addition of electrolyte were examined. The suspensions in de-ionized water without addition of acid, base, or electrolyte gave the largest apparent yield stress and shear viscosity, while the addition of base, acid, and KCl resulted in significant decreases of the apparent yield stress as well as of the viscosity. These effects have been interpreted in light of the DLVO theory and the compression of the double layer around the solid particles (Debye length). (C) 2003 The Society of Rheology.