Aqueous two-phase polymerization of acrylamide has been carried out in an aqueous solution of poly(ethylene glycol) initiated by ammonium persulfate. The chemical composition of the product was characterized by FTIR, and the droplet size distribution in the initial stage of the polymerization was followed by dynamic light scattering. On the basis of the evolution of polyacrylamide aqueous droplets size distribution and morphology at every stage, a new mechanism of droplet formation was proposed. The experimental phenomenon that the small droplets always existed in the process of polymerization and some irregular shape droplets were formed in the product of aqueous two phase polymerization could be successfully explained by the new mechanism. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 28592867, 2009