Pseudomorphic Si/Si-Ge strained layer superlattices are metastable and will relax to lower-energy, less strained, states on thermal annealing. Such relaxation may occur by the generation of misfit dislocation or by compositional homogenization of the superlattice. The particular mechanism adopted is shown to depend on the initial dislocation density of the structures. In cases where a significant portion of the strain is accommodated by an array of misfit dislocations in the as-grown state there is a propensity to relax by generating additional misfit dislocations. In the case of structures where only a very small fraction of the misfit is relieved by dislocations in the as-grown state, additional relaxation does not involve the multiplication of dislocations but proceeds by the interdiffusion of Si and Ge towards the homogenizing of the superlattice structure. This strain-enhanced diffusion has previously been observed in metals and we confirm its existence in semiconductor systems as well. The implication of the above observations on device structures and the growth of such layers is discussed.