Nitroxide-mediated radical polymerizations (NMP) of styrene at 100 degrees C using the cationic emulsifier tetradecyltrimethylammonium bromide (TTAB) have been carried Out employing 2,2'-azoisobutyronitrile (AIBN) and two nitroxides of different water solubilities. The polymerization rate was lower and the molecular weight distribution more narrow in microemulsion than bulk at low conversion. The results can be rationalized based oil the lower initiator efficiency in microemulsion than bulk, presumably caused by the confined space effect (compartmentalization) on geminate termination of AIBN radicals, and possibly also the confined space effect causing an increase in deactivation rate. The extent of retardation relative to bulk was more severe for the less water-soluble 2,2,5-trimethyl-4-phenyl-3-azahexane-3-oxv (TIPNO) than the more water-soluble N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide(SG1) as expected based on more extensive nitroxide exit occurring in the case of SG1 than TIPNO. The results illustrate how effects of heterogeneity can profoundly influence the course of NMP in dispersed systems.