The re-entry rate in emulsion polymerization is reformulated by taking into account both the enter and exit processes together. The role that the free radicals play in these rate processes is used directly in the formulation. Based on the formulation, expressions are derived for the re-entry and exit rate coefficients, the average number of free radicals in a polymer particle, and the total free radical concentration in the aqueous phase. The model results require only the basic parameters that are determined in independent experiments and do not involve parameter fitting. The results are applicable to the zero-one system in which there is only one or no free radical in the particle that prevails in interval II of the polymerization. A criterion is developed for determining the applicability of the zero-one system. Particle size dependence of the exit rate coefficient is also given. The model results are compared with literature data and are shown to predict certain exit rate behaviour that has been inexplicable hitherto.