The effects of initiator concentration on the kinetics of methyl methacrylate (MMA) and n-butyl acrylate (n-BuA) dispersion polymerizations were studied by reaction calorimetry. It was found that increasing the initiator concentration increased the reaction rate as well as the particle size in both systems. Three stages were distinguished in the reaction rate profiles of both MMA and n-BuA dispersion polymerizations. However, the dependencies of the reaction rate (initial and maximum R-p) on initiator concentration differed significantly for the two systems. This was interpreted on the basis of partitioning of monomer and initiator between phases, resulting in shifts in the reaction loci with conversion as indicated by the evolution of the molecular weight distributions.