The influence of microstructure on the fracture toughness, unnotched fracture strength, Young’s modulus and fracture mechanism of polybutadiene-reinforced poly(methylmethacrylate)s was investigated. The Young’s modulus increased with the degree of dispersion of the polybutadiene phase. A core-shell microstructure with fine (congruent-to 1 mum) rubber particles gave the highest fracture toughness. Cavitation of the polybutadiene phase was the dominant toughening mechanism with the particulate rubber morphologies, although additionally localized plastic yielding was observed on the fracture surface of the material with a core-shell microstructure. The material with a lamellar-type rubber morphology exhibited a high fracture toughness as a result of the rubber phase redirecting the propagating crack to produce a rough fracture surface.