Unidirectional continuous-fiber metal matrix composites are known for high strength in the fiber direction but are weak in the transverse direction, due often to weak fiber-matrix bonds. The effects of environment (vacuum, air, etc.) and of thermal treatment (isothermal and cyclic) on the fracture behavior and occasional fiber splitting were investigated in several such materials. Fractography was done using a scanning electron microscope, and fiber-matrix interface chemistry was determined using a scanning Auger microscope. The extent of fiber pullout is often an indication of the strength of longitudinally fractured composites. When there is excessive thermal oxidation of the fiber-matrix interface in the Al matrix composites with graphite fibers, pullout and weakening occur. A second cause of significant strength loss was excessive carbide formation and frequent fiber notching, so that longitudinal failures exhibited close-cropped fiber breakage. In Ti matrix composites with SiC and B₄C/B fibers, cracks along the interface were observed for thermally fatigued specimens. Large amounts of oxygen were found on both sides of the fractured fiber interface when thermal fatigued in air. Similar effects were noted in a sulfur environment.