2,2,6,6-Tetramethylpiperidinyl-l-oxy (TEMPO)mediated radical polymerization of styrene in aqueous mini-emulsion at 125 degrees C using sodium dodecylbenzenesulfonate 4 and poly(vinyl alcohol), respectively, as colloidal stabilizers has been investigated. The particle size had a dramatic effect on the polymerization process. Decreasing particle size led to a markedly higher polymerization rate, but less control and a lower degree of livingness. For particles with diameters greater than approximately 170 nm, the polymerization behavior was essentially the same as in the corresponding bulk system. By varying the particle size within an appropriate range, it is possible to tune the polymerization such that the polymerization rate is increased while still maintaining reasonable control and livingness.