The effects of elastomer type on the morphology, flammability, and mechanical properties of high-impact polystyrene (HIPS)/polystyrene (PS)-encapsulated magnesium hydroxide (MH) were investigated. The ternary composites were characterized by cone calorimetry, mechanical testing, and scanning electron microscopy. Morphology was controlled with poly[styrene-b-(ethyleneco-butylene)-b-styrene] (SEBS) triblock copolymer or the corresponding maleinated poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS-g-MA). The HIPS/SEBS/PS-encapsulated MH composites exhibited separation of the filler and elastomer, whereas the HIPS/SEBS-g-MA/PS-encapsulated MH composites exhibited encapsulation of the filler by SEBS-g-MA. The flame-retardant and mechanical properties of the ternary composites were strongly dependent on microstructure. The composites with an encapsulation structure showed higher flame-retardant properties than those with a separation structure at the optimum use level of SEBS-g-MA. Furthermore, the composites with a separation structure showed a higher modulus and impact strength than those with an encapsulation structure. (C) 2008 Wiley Periodicals, Inc.