The kinetic mechanism of the high-conversion free-radically initiated copolymerization of acrylonitrile with methyl acrylate in dimethylsulfoxide has been examined by using real-time C-13 nuclear magnetic resonance (n.m.r.) spectroscopy to monitor individual monomer conversions. Computer simulation techniques have been used to generate model predictions for comparison with experimental data. It has been found that the kinetic behaviour of the copolymerization can be explained by using Walling’s kinetic model with phi = 1 over the complete range of conversion without the necessity to invoke the concept of the penultimate-unit effect. However, it has been found that the initiator efficiency decreases very significantly around 80% conversion, and this has a considerable impact on the experimentally observed reaction rates over the last 20% conversion of monomers. The mathematical equations used to describe the influence of the initiator efficiency on the copolymerization reaction are an extension of those which successfully described the kinetic behaviour of the monomers in high-conversion homopolymerization reactions.