The fracture behavior of composite rubber particle-toughened polymethylmethacrylate has been investigated over a wide range of test speeds, encompassing impact conditions. When the entanglement density of the matrix was increased and its glass transition temperature reduced by copolymerization, there were significant increases in the crack initiation and propagation resistance of the particle-toughened materials at low to intermediate speeds. At impact speeds, on the other hand, where crazing became the dominant matrix microdeformation mechanism in all the materials investigated, the fracture response of the copolymer matrix was closer to that of the polymethylmethacrylate homopolymer, and the toughening effect of the rubber particles was no longer effective in either case. This is discussed in terms of the onset of the matrix b transition, associated with the transition from shear to crazing, and the a transition of the rubber domains, both of which occurred in the temperature range immediately below room temperature in low frequency dynamic torsion measurements.