Nitroxide-mediated controlled free-radical polymerizations of n-butyl acrylate and styrene were performed in miniemulsion using, for the first time, a water-soluble SG1-based alkoxyamine as an initiator. For n-butyl acrylate, polymerizations exhibited all the features of a controlled system. In particular, the initiator efficiency was high, indicating an extensive entry in the monomer droplets of all of the oligoradicals generated in the aqueous phase. The efficiency, however, was not high for styrene homopolymerization, which was explained by a highly pronounced persistent radical effect, leading to a large proportion of dead oligomers, to a large concentration of free nitroxide, and thus to a too slow chain growth in the aqueous phase. Nevertheless, the addition of a very small amount of methyl acrylate strongly enhanced the entry rate, as a consequence of a favorable kinetic effect due to appropriate copolymerization conditions.