Core-shell-type microspheres with microphase-separated shells of polystyrene (PS) and poly(ethylene glycol) (PEG) (microsphere(block): molar ratio: PS/PEG 49.1/45.9 mol %; M-w: PS chain: 1.07 X 10(4), PEG chain 1.0 X 10(4); the ratio of arm numbers of PEG to PS: 1.0; microsphere(graft): molar ratio: PS/PEG 33.8/55.9 mol %; M-w: PS chain: 1.54 X 10(4), PEG chain 1.0 X 10(4), the ratio of arm numbers of PEG to PS: 2.55) were synthesized by crosslinking of spherical domains of poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(4-vinyl pyridine) (P4VP) of the microphase-separated films of poly(ethylene glycol)-block-poly(2-hydroxyethyl methacrylate)block-polystyrene triblock terpolymer (M-n: 2.18 X 10(4); molar ratio: PS 49.1 mol %, PHEMA 5.0 mol %, PEG 45.9 mol %) and polystyrene-block-[poly(4-vinyl pyridine)-graft-poly(ethylene glycol)] block-graft copolymer (M-n: 4.56 X 10(4); molar ratio: PS 33.8 mol %, P4VP 10.3 mol %, PEG 55.9 mol %; branch number of PEG: 2.55), respectively. The structures of microphase-separated films were investigated by transmission electron microscopy and small-angle X-ray scattering. The effects of the arm number ratio of PS to PEG and the total arm number on the stability of the water/benzene emulsion were investigated. The emulsion stability of oil in water was improved by using the microsphere synthesized with the microsphere(graft). (C) 2003 Wiley Periodicals, Inc.