An experimental methodology has been developed which combines a continuous-flow system and an electron spin resonance time sweep experiment to study the batch emulsion polymerization of methyl methacrylate (MMA). This system allows essentially continuous monitoring of the propagating free-radical concentration during the reaction. The polymerization was initiated by a redox initiator system at a relatively low temperature (45 degrees C). This technique has been used to study the effect of varying latex particle size on polymerization kinetics. This study demonstrates that latex particle size has a significant effect on the PMMA propagating radical concentration profile, which is hypothesized to be due to changing the balance of the radical termination reactions. This study also provides evidence for an inhomogeneous distribution of the propagating PMMA radicals in the latex particle. The magnitude of the proposed inhomogeneity depends on the size of the particles.