Nitroxide-mediated controlled/living dispersion polymerization of styrene in supercritical carbon dioxide (scCO(2)) has been performed successfully to high conversion using N-tert-N-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide (SG1) and a poly(dimethylsiloxane-b-methyl methacrylate) stabilizer at 110 degrees C. The molecular weight distributions were narrow (M-w/M-n = 1.12-1.43), and the number-average molecular weight (M-n) values agreed well with theory. A large excess of free SG1 was required to obtain satisfactory control in the dispersion polymerization, possibly due to SG1 partitioning. The critical degree of polymerization at which the polymer precipitates (J(crit)) was determined as 28 by visual observation using a novel approach. Polymerizations conducted in solution ( toluene) proceeded at a similar rate (similar to 20% faster) to those in scCO(2). The number of chains increased with conversion in both dispersion and solution, with the greater increase being observed in the dispersion system.