The addition of polymers to colloidal dispersions can alter their colloidal stability by forcing aggregation and, under some conditions, can result in macroscopic precipitation of the particles or formation of a particle gel. Here, we investigate the phase behavior of silica colloids in solution with two uncharged polymers, polyethylene oxide (PEO) and polyvinylpyrrolidone (PVP), with the values of Rg / R = 0.7 and 1.8 for each polymer. The polymer properties were characterized by viscometry, light scattering and size exclusion chromatography. An experimental complication is the potential adsorption of polymer to the surface of the particle. The PEO–silica system phase separates by depletion flocculation 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, the onset of phase separation was observed at polymer concentrations below silica surface saturation which indicates that PVP bridges two colloidal particles rather than causes a depletion effect. Depletion flocculation, however, sustains phase separation above surface saturation. In acidic buffer, depletion flocculation is the method of phase separation. This difference in separation mechanism for the PVP-silica system was investigated by quantifying the strength of interaction between PVP and silica from water and from buffer using isothermal titration calorimetry. Rheological creep tests were able to differentiate between samples that separate by depletion only, bridging only, and those that bridged before depletion.