A novel approach is presented whereby nitroxide-mediated radical polymerization (NMP) is conducted in an aqueous heterogeneous system at an initial polymerization rate an order of magnitude greater than the corresponding bulk system, accompanied by an improvement in the level of control over the molecular weight distribution. NMP of styrene mediated by N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide (SG1) at 90 °C was performed in a miniemulsion with exceptionally small particles of number-average diameter ∼10 nm, generated by ultrasonication combined with in situ surfactant formation. The results are discussed in terms of the effects of compartmentalization, nitroxide partitioning (exit/entry), and a rate-enhancing effect of oleic acid. These findings illustrate that it is possible to significantly improve the performance of an NMP process by the exploitation of intrinsic effects of heterogeneous systems.