The influence of ammonium chloride (NH4Cl) on the rheological properties and sedimentation behavior of aqueous silica (SiO2) suspensions of varying solids volume fraction (phi(s)) was studied. SiO2 suspensions with low NR4Cl concentration (less than or equal to 0.05M, pH 5.2) exhibited Newtonian behavior and a constant settling velocity (U). The volume fraction dependence was well described by the Richardson-Zaki form, U = U-0(1 - phi(s))(n), where n = 4.63 and U-0 = 1.0419 x 10(-5) cm/s. At higher NH4Cl concentration (0.07-2.0M, pH 5.2), suspensions exhibited shear thinning and more complicated sedimentation behavior due to their aggregated nature. For all suspensions studied, however, the apparent suspension viscosity, characteristics cluster size, and initial settling velocity were greatest at similar to 0.5M NH4Cl and exhibited a similar dependence on salt concentration. Above 0.5M NH4Cl, considerable restabilization was observed. This behavior cannot be explained by traditional DLVO theory.