Sedimentation not only controls some important industrial processes but also is a readily available tool for particle separation, determination of particle size and particle size distribution. Velocity of sedimentation is a controlling parameter in this process. The hydrodynamic forces of the system, interactions of particles with matrix and with each other control this parameter. In highly filled suspensions, particle-particle interactions affect sedimentation more than the hydrodynamic and particle-matrix interaction. Existence of polymer in the matrix would change matrix viscosity, affecting hydrodynamic motion of particles; and adsorption of polymer on the surface of particles would change surface properties, causing alteration in particle-matrix and particle-particle interactions. This work is a study of effect of polymer concentration on particle-particle interactions. The results show that at low particle concentrations, addition of a small amount of polymer in the solution would decrease particle-particle interactions. However, in a highly filled suspension, polymer does not affect particle-particle interactions. Toluene was used as the solvent, nano-silica as the particle and polystyrene as the polymer. A high shear mixer was used to reduce the agglomeration of nano-silica particles, especially for highly filled suspensions. Sedimentation tests were performed in columns with a length of 50 cm and 26 mm in diameter. (C) 2011 Elsevier B.V. All rights reserved.