The thermal stability of high quality, strained Si1-xSnx, 2.5%≤x≤5%, grown by molecular beam epitaxy on Si <001> substrates has been investigated by Rutherford backscattering spectrometry and transmission electron microscopy. As a result of annealing at temperatures in the range 400–950 °C for 1 h, both relaxation by precipitation of Sn and generation of misfit dislocations and dislocation loops were found, consistent with the low solubility of Sn in Si and the large lattice mismatch between Si and Si1-xSnx. In the epitaxial, strained Si1-xSnx layers, where the strain is proportional to the Sn concentration, the threshold temperature for generation of precipitates and misfit dislocations was found to decrease with increasing Sn concentration. Above the threshold temperature the influences of the different relaxation channels change considerably with temperature and composition; an orthogonal interfacial misfit dislocation network is seen at temperatures close to threshold while at higher temperatures mainly dislocations in the bulk are found coexisting with large precipitates. The compositional metastability that leads to the precipitation process, reduces the Sn concentration in the matrix, however, neither this depletion of Sn from the matrix of Si1-xSnx nor the relaxation due to misfit dislocations cause a fully relaxation of the Si1-xSnx layers. © 1998 American Vacuum Society.