The viscosity behavior of aqueous suspensions of silica particles with physically adsorbed polyethylene oxide (PEO) molecules was investigated in relation to the adsorbed layer density and concentration of nonadsorbed polymer in the suspending fluid. Adsorption studies revealed the presence of two plateaus in the adsorption isotherm, which was attributed to a change in conformation from a flat, "pancake"-type, to a more elongated "brush"-type conformation of the adsorbed polymer, where the terminal hydroxyl groups remained bound to the silica surface. Adsorption and rheological studies were performed on suspensions of silica particles of three different sizes dispersed in solutions of 0.01 M NaNO 3 containing PEO of different concentrations and different molecular weights. The variation of shear viscosity with the adsorbed layer density, concentration of free polymer in the suspending media (depletion forces), and particle size are discussed. Results on the role of particle size on the viscosity of electrostatically and sterically stabilized suspensions indicate that sterically stabilized systems may be treated as hard spheres at high shear rates depending upon the adsorbed layer density and concentration of nonadsorbed polymer in the suspending media. Electrostatically stabilized suspensions may show deviation from hard sphere behavior even at high shear rates depending on the range of electrostatic repulsion between the suspended particles. Further investigation must be performed to be able to explain the difference between the behavior of electrostatically and sterically stabilized systems in terms of the deformability of the layers, i.e., "softness" of the electrical double layers around the particles as compared with the "stiffness" of the adsorbed layers of the polymer.