Tension specimens of boron reinforced 7075 and 6061 aluminum were tested at angles of 0, 30, and 90 deg to the unidirectionally aligned filaments. Metallurgical structures in the matrix and at interfaces were correlated with stress-strain curves, with acoustic emission from boron filament breakage during the tests, and with fractographic studies. Differences in composite strength after various heat treatments were attributed to changes in the properties of the matrix rather than to any difference in properties of the filament-matrix interface. Transmission electron microscopy of boron/ 7075 aluminum composites revealed (1) precipitate segregation to interfaces for all heat treatments, (2) that subzero thermal treatments provide a new means of developing desired microstructures in composites, and (3) the presence of a narrow surface layer in the filaments in boron/7075 aluminum composites. This surface layer is believed to be a result of diffusion during composite fabrication.