A detailed study was conducted of the microstructure and particle-matrix interfaces in Al/TiCp metal matrix composites prepared by the XD process and subsequent extrusion. A study of the morphology of the TiC particles showed that the surfaces are low index (111) and (200) planes, the former being more common. Direct contact on an atomic scale is established between Al and TiC, allowing chemical bonds to form. Young's modulus is in the range expected for a composite of Al and TiC with good interfacial bonding and load transfer to the particles. No third element has been detected at the interfaces, showing that they are clean. Both incoherent and semicoherent interfaces are seen. The interface character depends on the size of the particles and their orientation with respect to the neighboring Al grains. “Special” interfaces with evidence for nearly periodic dislocations were observed in both XD Al/TiC and Al/TiB2 composites, indicating the general tendency of in situ composites to lower their interfacial energy by forming such boundaries.