Block copolymers containing either poly(propylene oxide) (PPO) or poly(butylene oxide) (PBO) stabilizer group(s) and a poly(ethylene oxide) (PEO) anchor group prevent flocculation of amorphous poly(methyl methacrylate) (PMMA) microparticles formed by spraying PMMA solutions into flowing liquid CO2 at 23 degrees C. When dissolved PPO-PEO-PPO triblock and PBO-PEO diblock copolymers are introduced with the CO2 feed stream, 0.1-0.5 mu m primary PMMA particles are produced. However, larger, and in some cases more spherical microparticles (0.5-2.0 mu m) are formed when these stabilizers are fed via the polymer solution phase, for the same overall quantity of stabilizer. The effectiveness of the stabilizer is described in terms of its concentration and how it partitions between the dispersed phase, the interface, and the CO2 phase. In many cases stabilizers with only moderate solubilities in CO2 are more effective than those with higher or lower solubilities. When the stabilizer is introduced with the solution phase, it does not have to be soluble in CO2 to prevent flocculation. The latex particle size, stability, critical flocculation density, and reversibility of flocculation have been measured in-situ by turbidimetry to understand the mechanism of steric stabilization in supercritical fluids. The size of the primary particles in the product determined by scanning electron microscopy is consistent with in-situ measurements of particle size by turbidimetry.