Mass transfer resistance in the production of high impact polypropylene (hiPP) produced by a two-stage slurry/gas polymerization was investigated by field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. It is found that the formation of ethylene-propylene copolymer (EPR) phases in polypropylene (iPP) particle produced in the first stage slurry polymerization exhibits a developing process from exterior to interior. During the early stage of ethylene-propylene copolymerization, with lower content of copolymerized ethylene (7.4 mol%), the EPR phases occur only in the external layer of the particle, while at the later stage of the copolymerization with higher content of copolymerized ethylene (26.7 mol%), the elastomer phases distribute uniformly in the whole particle. This phenomenon is due to an effect of mass transfer resistance. The origin of mass transfer resistance is loosely agglomerate inclusions of low tacticity polypropylene within the semi-filled micropores inside the iPP particle. It is the inclusions inside the micropores that resist the diffusion of ethylene/propylene comonomers into the particle. (C) 2008 Elsevier Ltd. All rights reserved.