Aqueous solutions of silica with PEO or PVP at Rg/R=0.7 and 1.8 were investigated. An experimental complication is the potential adsorption of polymer to the surface of the particle. The PEO–silica system can be modeled with depletion theories because phase separation occurs at polymer concentrations above silica surface saturation. On the other hand, phase separation for the PVP–silica system depends on the choice of buffer. In DI water, separation was observed at polymer concentrations below silica surface saturation which indicates that PVP bridges two colloidal particles rather than causes a depletion effect. In acidic buffer, depletion flocculation is the method of phase separation hence, depletion models should be appropriate. The ratio of polymer concentration to polymer entanglement concentration, c/c*, increases with increasing Rg/R for all four polymer–particle systems in buffer. This trend opposes classical depletion theories but is in agreement with the PRISM theory of athermal polymer–colloid suspensions.