Strain relaxation in Si1−xGex/Si superlattices and alloy films is studied as a function ofexsituanneal treatment with the use of x‐ray diffraction and Raman spectroscopy. Samples are grown by molecular‐beam epitaxy at an unusually low temperature (≊365 °C). This results in metastably strained alloy and superlattice films significantly in excess of critical thicknesses previously reported for such structures. Significant strain relaxation is observed upon anneal at temperatures as low as 390 °C. After a 700 °C, 2 h anneal, superlattices are observed to relax less fully (∼43% of coherent strain) than corresponding alloys (∼84% of coherent strain). Also, the strain relaxation kinetics of a Si1−xGexalloy layer is studied quantitatively. Alloy strain relaxation is approximately described by a single, thermally activated, first order kinetic process having activation energyEa=2.0 eV. The relevance of our results to the microscopic mechanisms responsible for strain relaxation in lattice‐mismatched semiconductor heterostructures is discussed.