Nanocapsules (below 100 nm) with liquid cores and molar mass controlled polystyrene shells were synthesized by an in situ miniemulsion polymerization reaction in the presence of a RAFT (reversible addition fragmentation chain transfer) agent. The formation of structured particles with the targeted core/shell morphology, i.e. liquid cores and polymeric shells, is extremely dependent on the type of RAFT agent used in conjunction with the type of initiator used. Different RAFT agents lead to different polymerization rates, thus resulting in different chain lengths as a function of time. This will influence viscosity and consequently chain mobility and can therefore cause a deviation from the desired morphology. The type of initiator used influences the surface activity of entering oligomers and is therefore also an important factor in obtaining the correct structure. Results showed that a RAFT agent that causes no rate retardation (phenyl 2-propyl phenyl dithioacetate, PPPDTA), used in conjunction with a surface active initiating species (potassium persulfate, KPS), is able to lock the locus of polymerization at the droplet/water interface. This results in entering oligomers being anchored at the droplet/water interface with consequent core/shell (nanocapsule) formation only if the RAFT agent used leads to a sufficiently rapid increase in chain length with time and thus a restriction of chain mobility of the mediated species. © 2005 Elsevier Ltd. All rights reserved.