Interfacial microstructure can have a significant influence on the microfracture processes of discontinuous reinforced metal matrix composites (DMMCs). The fracture properties, however, are largely influenced by the microfracture and deformation mechanisms associated with the matrix microstructure and with the interface microstructure. Also, it is known that the paniculate morphology and distribution can modify the deformation process by influencing the stress state that develops in the matrix materials near the reinforcement. Along with the matrix microstructure, characterizing the role of the interfacial and the near-interfacial microstructure is essential for a broader understanding of fracture behavior in DMMCs. To characterize the microstructure of cast DMMCs, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and electron microprobe (EMP) examinations were conducted at the interface and in regions near the particulate/matrix interface. Materials studied consisted of cast Al-4.5Cu and Al-7Si matrix alloy systems with B4C and SiC reinforcement. In general, the interfacial and matrix microstructure of Al-4.5Cu/SiC and Al-4.5 CuB4C composites exhibited little variance, i.e. the reinforcement type had no apparent effect on the resultant microstructures. For the Al-7Si system, however, significant microstructural variance was observed both in the matrix and interfacial regions. In the Al-7Si/B4C composite, an extensive reaction zone was found at the B4C interface. Interfacial compounds observed in Al-7SiC/B4C were Ti(O,B), Si, and MgB6 precipitates. In the near-interface region compounds such as Alx(B, C, 0)y, AlxMg(1−x)B2, and Al4C3 were found. In sharp contrast to Al-7SiC/B4C, an extensive interfacial reaction zone was not revealed for Al-7Si/SiC MMC. Only isolated, extremely fine second phase precipitates were observed on SiC paniculate interfaces. Fracture surface evidence suggested that both the matrix and the interface microstructure influenced deformation and microfracture mechanisms in DMMCs.