Microcapsules with narrow size distribution, in which hexadecane (HD) was used as a oily core and poly(styrene-co-NN-dimethylamino-ethyl methacrylate) [P(St-DMAEMA)] as a wall, were prepared by a Shirasu porous glass (SPG) emulsification technique and a subsequent suspension polymerization process. That is, a mixture of St monomer, DMAEMA monomer, HD, and NN'-azobis(2,4-dimethylvaleronitrile) initiator was permeated through the uniform pores of an SPG membrane into a continuous phase containing a poly(vinylpyrrolidone), sodium lauryl sulfate, and sodium nitrite water-soluble inhibitor by applying a pressure, to form uniform droplets. Then, the droplets were polymerized at 70 degreesC. It was found that HD was encapsulated completely only when conversion was quite high, irrespective of whether a DMAEMA hydrophilic monomer was incorporated into the polymer. As the amount of HD increased, HD was more easily encapsulated by the polymer. In order to clarify the reason for such unique behavior, a simulation was carried out, taking the St monomer partition in the HD phase and PSt wall phase into the consideration. It was found that the main reason HD could not be encapsulated completely by PSt when conversion was lower was that the interfacial tension of the HD phase with water and the PSt phase with water got closer. As a result, both HD and PSt can come in contact with the water phase. (C) 2003 Elsevier Inc. All rights reserved.