The effect of the of the reactive modifier 1,3-phenylene dimaleimide (BMI) on the interphase structure of poly(propene) block copolymer (PPBC)/magnesium hydroxide (Mg(OH)(2)) composites has been investigated. DRIFTS and DSC studies on hot decane extracted composites confirmed preferential encapsulation of the filler particles with the ethene based elastomeric phase of the PPBC. This elastomeric interphase structure, formed efficiently via reactive processing of untreated Mg(OH)(2), PPBC and BMI, conferred excellent strength and toughness to the composites. The tensile strength and elongation of a BMI modified composite containing 60% w/w Mg(OH)(2), exceeded the yield stress and strain of the unfilled matrix.