Steric stabilization and flocculation of colloids with surface-grafted poly(dimethylsiloxane) (PDMS) chains are examined in liquid and supercritical carbon dioxide with and without hexane as a cosolvent. Neither poly(methyl methacrylate) (PMMA) nor silica particles with grafted 10,000 g/mol PDMS could be stabilized in pure CO2 at pressures up to 345 bar at 25 degrees C and 517 bar at 65 degrees C without stirring. The addition of 15 wt% hexane to CO2 led to stable dispersions with sedimentation velocities of 0.2 mm/min for 1-2 mu m PMMA particles. The critical flocculation pressure of the colloids in the hexane/CO2 mixture, determined from turbidity versus time measurements, was found to be the same for silica and PMMA particles and was well above the upper critical solution pressure for the PDMS-CO2 system. The addition of a nonreactive cosolvent, hexane, eliminates flocculation of PMMA particles synthesized through dispersion polymerization in CO2 with PDMS-based surfactants.