Comparison of high resolution x-ray diffraction and transmission electron microscopy measurements of Si-based heterostructures demonstrates that diffraction is much more sensitive to strain relaxation than previously reported. This study used as-grown and annealed Si1-x Gex structures grown on (001) Si by UHV/CVD. (004), (113), and (115) rocking curves were employed. Using the TEM measurements as a quantitative guide, relaxation was observed in rocking curves when the misfit dislocation line density was as low as 1 μ-1. Also, interference fringes strongly depend on the presence of interfacial defects. At higher dislocation densities, the diffraction peak from the epitaxial layer broadens considerably but does not shift to a position that represents complete relaxation. Broadening of the substrate diffraction peak also occurs, which is due to dislocations that loop into the substrate.