The rheological properties of both aqueous and non-aqueous composite suspensions were investigated together with a characterization of the colloidal stability of the separate components. The colloidally stable, concentrated aqueous SiCw and Al2O3-SiCw composite suspensions displayed strong sheat thinning followed by a severe, sometimes discontinuous, shear thickening at a critical sheat rate. The non-aqueous composite suspensions, containing weakly flocculated SIG(w), only showed a continuous sheat thinning behaviour. The variations in the steady shear behaviour were related to the differences in the colloidal stability and possible sheat induced effects on the suspension structure. It was possible to fit the volume fraction dependence of both the SiCw and the Al2O3 suspensions to a modified Krieger-Dougherty model which yields values of the maximum volume fraction; phi(m) (Al2O3) = 0.61, phi(m)(SiCw) = 0.28. The viscosity of the composite suspensions were successfully predicted from the Farris theory using the rheological data for the separate components.