A model for the evaluation of the kinetics and the chain length distribution in living/controlled radical polymerization mediated by reversible addition-fragmentation chain transfer (RAFT) in bulk is presented. Using the free volume theory, the model accounts for the diffusion limitations over both termination and RAFT exchange reactions. Model predictions are compared to experimental results of methyl methacrylate polymerization with cumyl dithiobenzoate as a RAFT agent. It is shown that the polymerization retardation observed in living systems at large conversions is well predicted. (c) 2005 Wiley Periodicals, Inc.