Micrometer-sized poly(methyl methacrylate) (PMMA) particles were successfully prepared without submicrometer-sized by-products for the first time by applying microsuspension iodine transfer polymerization (ms ITP) with benzoyl peroxide as an initiator and iodoform as a chain transfer agent in aqueous dispersed systems. Iodine radicals must react almost immediately after their exit from the monomer droplet, because of their high insolubility in water; we have named this phenomenon the radical exit depression (RED) effect. In contrast, both sub-micrometer and micrometer sized PMMA particles, which are formed based on homogeneous and droplet nucleations, respectively, were obtained by microsuspension conventional radical polymerization (ms CRP). Formation of submicrometer PMMA products as a by-product of ms CRP has been a serious problem for a long time to be overcome in the synthesis of micrometer sized polymers.