Silicon carbide and 6061 aluminum alloy possess very different thermalexpansion coefficients; 3-3 and 22.5.10-6K-1, respectively. Thus, one expects large internal strains and stresses in these composites because the two constituents form interfacial bonds at high temperatures and are cooled to ambient temperatures. From a simple elastic model, one expects a hydrostatic tensile stress in the aluminum matrix and a-hydrostatic compressive stress in the silicon-carbide particles. Using conventional diffraction geometry, using Cu Kα radiation, we studied three surfaces of a plate specimen. For both phases, we determined the unit-cell dimensions for two situations: unmixed and mixed in the final composite. The silicon-carbide particles showed a compressive stress and the aluminum matrix a tensile stress, seventy-five percent of the yield strength. Measurements show that both stress tensors are approximately hydrostatic.