In a recent note (1) to this journal the preparation of nonaqueous dispersions of small, spherical silica particles was described, in which the particles were stabilized by polystyrene (PS) chains, terminally anchored to the silica surface by an anionic polymerization technique. It was suggested that these systems could form useful models for nonaqueous particulate dispersions, since the particle cores were nonswelling and the stabilizing polymer chains were of controlled molecular weight and attached to the surface in a well-defined configuration (i.e., tails). Moreover, the degree of coverage of the surface could be varied. In this paper we report the use of SiO2-g-PS dispersions of this type to investigate the effect of added polymer on dispersion stability. In a number of publications (2-6) from this laboratory it has been shown that the addition of polymer, over a certain concentration range, can lead to instability in the form of weak, reversible flocculation. In addition, we have shown (6, 7) that the phase separation behavior shown by polystyrene microgel dispersions in the presence of added polymer and first reported by Sieglaff (8), is a closely related phenomenon. Moreover, both the stability of aqueous particulate dispersions (4) and of nonaqueous microgel systems (7) in the