The work described here has shown for the first time that well-defined core-shell nanostructures with controlled particle size and molecular weight distributions (in which the molecular weight distributions of both blocks are independently controlled using the same RAFT controlling agent) can be prepared in an aqueous environment. These types of nanoparticles can lead the way to more novel and interesting structures in applications for drug delivery, biosensors, and other devices. A poly(methyl methacrylate) (PMMA) seed was used to control the particle size distribution, in which styrene (STY) in the presence of the RAFT agent was polymerized under batch conditions to give polymer particles with an number-average molecular weight (M-n) of 7000 and polydispersity (PDI) of 2. Butyl acrylate (BA) was then polymerized under semibatch conditions into these particles to give an overall Mn of 20 000 and PDI close to 1.3 for the block copolymer, in which greater than 90% block purity was observed. The calculated PDI of the second block (PBA) was close to 1.4, which is lower than the theoretically determined value under batch conditions (PDI = 1.6). The results show that by slow monomer addition feed into the reactor the PDI can be reduced and that by using the advantage of "random coupling" between the two blocks the PDI of the final block copolymer is lower than either of the individual blocks.