Electron spin resonance (ESR) spectroscopy has been employed to monitor the occluded radical concentration during the batch (unseeded) emulsion copolymerization of methyl methacrylate and butyl acrylate. Occluded radical concentrations, above the detection limit of the ESR spectrometer, are only observed at methyl methacrylate mole fractions >0.96. The occluded radicals reach a maximum concentration at high conversion (ca 95%) and thereafter their concentraion declines via a second order termination process. The variation in the maximum reaction temperature and the time at which this maximum occurs with mole fraction has also been monitored and is discussed. The most surprising feature of these latter observations is that the rate of polymerization is much faster for butyl acrylate than for methyl methacrylate despite the greater propagation rate constant of the latter.