SixGe1−x strained layer superlattices are known to be metastable in that they can be grown fully commensurate with layer thickness higher than the equilibrium, calculated Tc at which dislocation formation becomes energetically favorable. In this paper, we describe the mechanism of relaxation in such multilayers. Both plane-view and cross-sectional transmission electron microscopy (TEM) were used to examine the formation of dislocation at the different interfaces. RBS was used to follow interdiffusion. We found two competing mechanisms for relaxation: The preferred mode for relaxation is the creation of dislocation networks at each of the interfaces. This process can be stopped or considerably inhibited by the difficulty of forming new dislocations in samples which are perfectly commensurate after growth; Some dislocations appear necessary in order to generate more dislocations during annealing. When this is not the case, the only possible way to attain relaxation is through diffusion. In such a case, stress-enhanced diffusion is observed, with a diffusion coefficient 200 times higher than expected.