TEMPO-mediated styrene miniemulsion polymerizations were conducted at varying particle sizes (similar to 50, 90, and 180 nm) to study possible effects of compartmentalization. Polymerizations were initiated using a TEMPO-terminated polystyrene, macroinitiator that also acted as costabilizer for the miniemulsion. A bulk polymerization was conducted as a control. Conversion, molecular weight distribution, and the chain livingness were measured to assess the effects of particle size. Decreasing particle size resulted in lower rates of polymerization; after 6 h of polymerization, conversions were 59% for similar to 180 nm particles and 43% for similar to 50 nm particles. More importantly, large differences in the polymer chain livingness were observed, with smaller particles exhibiting superior livingness at equivalent conversions. Minor effects were also observed on molecular weight; decreasing particle size resulted in higher M-n at a specified conversion, signifying lower chain concentrations in smaller particles.