An atomic force microscope has been used to study the interaction forces between spherical silica particles (of radius 3-6 mu m) and a planar silica surface mediated by aqueous solutions of sodium poly(styrenesulfonate). The experimental parameters explored were the free polymer molecular weight and concentration, the background electrolyte concentration, and pH. Force-distance curves displayed a short-range repulsion due to overlap of opposing electrical double layers and then a minimum due to depletion of the polymer. In some instances, at further surface separations, oscillatory forces indicative of polymer coil structuring were observed for both approaching and retracting surfaces.