Nitroxide-mediated polymerizations of styrene in microemulsion have been carried out at 125 degrees C using the cationic surfactant tetradecyltrimethylammonium bromide and the nitroxides 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) and N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide (SG1). TEMPO-mediated polymerizations were extremely slow, with large particles (d(n)=39-129 nm) and broad molecular weight distributions (MWDs). The origin of the broad MWDs was likely significant alkoxyamine decomposition and differing diffusion rates of monomer and low MW alkoxyamines (and nitroxide) between monomer-swollen micelles and polymer particles. SG1-mediated polymerizations proceeded at higher rates, resulting in nanoparticles (d(n)= 21-37 nm) and lower (M) over bar (w)/(M) over bar (n) than for TEMPO.