The impact properties of advanced metal matrix composites (MMCs) such as unidirectionally reinforced silicon carbide (SiC)/aluminum (Al) and C/Al have been investigated based on recent improvements in their mechanical properties. This paper details the effects of test temperature and neutron irradiation on the fracture toughness of these MMCs. The materials used were sheets of PCS-SiC/Al and PAN-C/Al. Miniaturized Charpy V-notched specimens were tested in an instrumented Charpy impact tester. Neutron irradiation was performed in the Japanese Materials Testing Reactor (JMTR), a light water reactor (LWR) at Oarai. The fracture energy increased with increasing test temperature and with neutron irradiation. SiC/Al was rather more insensitive to neutron fluence than C/Al, which was related to the difference in interfacial structure between the two systems.