The rheological behavior of concentrated nearly monodisperse, nearly spherical silica, and narrow particle size distribution alumina suspensions was measured in Couette geometry. Attractive forces between the particles (suspensions flocculated at or near the isoelectric point) produce high viscosities. Repulsive forces between the particles produce dispersed suspensions and decrease the viscosity at low and intermediate shear rates. The viscosity of the dispersed suspensions shear thickens (abruptly increases and approaches that of the flocculated suspensions) at a critical shear rate land therefore stress) provided the volume fraction of solids of the suspension is sufficiently high. As the pH is adjusted farther from the isoelectric point the critical shear rate land stress) increase. At pH away from the isoelectric point the addition of salt decreases the critical shear rate land stress). Thus shear thickening is not only dependent upon hydrodynamic interactions but also depends on surface forces, in particular repulsive interparticle forces. Increasing the magnitude and range of the repulsive forces increases the shear stress land rate) at which shear thickening starts. Reducing the magnitude of the repulsive force allows the particles to be more easily forced into clusters resulting in shear thickening.