A detailed model describing the kinetics of living polymerization mediated by reversible addition-fragmentation chain transfer (RAFT) in seeded emulsion polymerization is developed. The model consists of a set of population balance equations of the different radical species in the aqueous phase and in the particle phase (accounting for radical segregation) as well as for the dormant species in the particle phase. The entire population of radicals was divided into several distinguished species, based on their length and their chain end group. The model results are helpful in understanding inhibition and retardation phenomena that are typical for RAFT emulsion polymerizations. While inhibition is due to the radical loss in form of the RAFT leaving group, retardation is mostly caused by a small amount of short dormant chains in the particle phase, leading to a slight increase of radical loss via RAFT exchange with radicals entering a particle. The model results are compared to a series of experiments, using cumyl dithiobenzoate as a RAFT agent in polymerizations of styrene. The agreement between experimental and model results is good and, notably, the only parameters considered adjustable were the RAFT exchange rate coefficients. (c) 2006 Wiley Periodicals, Inc.