The macro- and miniemulsion polymerization of butyl acrylate (BA) with cumene hydroperoxide/tetraethlenepentamine (CHP/-TEPA) as a redox initiator system was investigated. It was found that the rate of polymerization was monotonically decreasing rather than going through a maximum as is common in emulsion polymerization. Furthermore, the polymerization rate at high monomer conversion of macroemulsion polymerization was unexpectedly decreased with an increase in initiator concentration. For miniemulsion polymerization, the polymerization stopped at rather low conversion. It was also found that the average number of free radicals per particle dropped very quickly with polymerization time. With increased initiator concentration, the average number of radicals per particle decreased in the macroemulsion polymerization. Molecular weight analysis indicated that aqueous phase polymerization, interfacial polymerization and particle polymerization take place simultaneously. It is proposed that TEPA plays two roles in the polymerization; it is both a reducing agent in the redox initiator system, and a chain transfer agent. All the unexpected results are well explained by postulating this multiple function of TEPA and taking the heterogeneous nature of emulsion polymerization into account.