Screw-extruded polymer blends of poly(styrene-block-butadiene-block-styrene) triblock copolymer (KR05) with methyl methacrylate-styrene copolymer (MS-200) were injection moulded, and the mechanical properties and morphology were examined. KR05 is toughened by incorporation of MS-200, and synergistic improvement of toughness is observed for blends of KR05 matrix with MS-200 inclusions. Under tensile stress the KR05 matrix of the injection-moulded blends shows the same structural change as injection-moulded KR05 (extensive shear yielding). However, the blends also show cavitational breakdown in the polybutadiene phase and numerous outbreaks of minute debonding in the interfacial region between the KR05 matrix and MS-200 particles. These microscopic tearing phenomena could restrain excessive stress concentration and dissipate large energy before macrocrack propagation, leading to synergistic toughening.