The stability of silica dispersions (Ludox HS) has been studied in the presence of two nonadsorbing polyelectrolytes, i.e., sodium carboxymethyl cellulose (CMC) and sodium polystyrene sulfonate (SPSS). For CMC, phase segregation occurred and polymer- and particle-rich plases were formed. The particle phase retained its liquidlike properties. Increasing the polymer charge density reduced the tendency for phase separation to occur while varying the molecular mass had little effect. For SPSS, particle flocculation/gelation occurred rather than polymer/particle segregation and increased polymer concentrations were required. The concentration of CMC required to induce separation increased in the presence of (0.1 M \mathrm{NH}{4} \mathrm{NO}{3}) while that of SPSS decreased. Particle interaction in the presence of CMC was monitored by turbidity mcasurements and the second virial coefficients obtained were in accordance with the trends observed in the phase behavior. Osmotic pressure and sodium ion activity measurements indicated that additional counterion condensation occurred on mixing of the polymer and particles. Potential energy calculations incorporating van der Waals, electrostatic, and depletion terms indicated that CMC will induce phase separation more readily than SPSS due to its increased depletion force and hence could explain the differences in behavior observed between the two polymers. The fact that CMC molecules are relatively stiff compared to SPSS molecules and thus can form concentrated anisotropic phases is also believed to be an important factor. Although depletion theory predicted that phase separation would occur more readily as the CMC charge density increased, the reverse trend was observed and was attributed to increased intermolecular repulsion between the CMC chains. &i 1994 Acadenic Priss. inc.