To enhance their properties, melt processed polymers are commonly filled with colloidal particles. Dispersing particles homogeneously in a melt is generally difficult, particularly with dense inorganic oxides that generate strong van der Waals attractions. These attractions can be modulated by introducing repulsive forces through surface modifications such as polymer grafting. Indeed, the relative viscosity of 430 nm alumina particles stabilized by end-tethered poly(dimethylsiloxane) (PDMS) in PDMS melts decreases with increasing graft density and molecular weight (50 or 31.4 kg/mol) as expected, but also with increasing molecular weight of the melt in the range of 4.7-41.1 kg/mol. This is surprising, as well established theory predicts the grafted layer to be more swollen and, therefore, a better stabilizer in lower molecular weight melts. The answer is found in other studies showing that PDMS melts adsorb on alumina, providing a steric barrier that increases with melt molecular weight. A simple two-parameter correlation fits the shear-rate dependent viscosities with a relative high shear viscosity and a structural relaxation time that capture the hydrodynamic volume and the interparticle forces, respectively. The volume fraction dependence of the high shear viscosity indicates the degree of permanent clustering. The measured relaxation time can be correlated reasonably well with a characteristic relaxation time estimated by balancing the viscous forces in the gap against the van der Waals forces, both of which depend on a characteristic separation reflecting the thickness of the combined adsorbed and grafted layer. (C) 2011 The Society of Rheology. [DOI: 10.1122/1.3539999]