We report a double‐crystal x‐ray diffraction study of the relaxation of molecular beam epitaxy grown Si‐Si1−xGexstrained single layers and superlattices on (100) Si. The thermal stability of the heterostructures was investigated by annealing between 600 and 900 °C. Measurement of (400) rocking curves demonstrated that all the heterostructures were initially coherently strained and had excellent crystallinity. Upon annealing deterioration of the crystal quality and progressive relaxation was observed on some of the samples while on others no relaxation or loss of crystalline quality was detected. These observations are consistent with the mechanical equilibrium theory predicting the critical thickness for pseudomorphic growth of lattice mismatch materials. However, the concept of critical stress needs to be invoked to account for the stability of dilute Si1−xGexalloy layers on Si.