Herein we report the use of poly(styrene-b-dimethylsiloxane) block copolymers as steric stabilizers for dispersion polymerizations of styrene in a carbon dioxide continuous phase. It was demonstrated that the anchor-soluble balance (ASB) of the stabilizer has a dramatic effect on both the progress of the reaction and the morphology of the resulting polystyrene (PS) colloid. When a stabilizer with an appropriate ASB is employed, the initial concentrations of styrene, stabilizer, and helium have substantial effects on the resulting size and dispersity of the colloidal particles. Dispersion polymerizations carried out under a series of different pressures (143-439 bar) resulted in variations in the conversion, molecular weight, and PS particle diameter, suggesting that the reaction is extremely sensitive to the density of the continuous phase. An investigation of the progress of the reaction as a function of time at. two different temperatures revealed that the average molecular weight increases gradually over the course; of the reaction. Finally, control reactions in heptane solvent were not as successful as those conducted. in CO2 suggesting that the plasticization of the polymer particles by CO2 facilitates diffusion of monomer to the growing chain ends.