The relaxation of variable thickness strained silicon layers on 20% and 50% germanium composition virtual substrates have been quantified using two independent methods. High resolution X-ray diffraction offers a means to measure relaxation directly, and a defect etching technique has been developed from which relaxation can be determined by the measurement of dislocation densities. Comparisons between the relaxation of tensile strained silicon in this work and compressively strained Si1?xGex in other works, suggest that strained silicon is unusually stable to relaxation. Observation of dislocation structures with defect etching and transmission electron microscopy have shown that the additional stability arises from the interaction of dislocations which inhibits glide. Extended stacking faults, which only form when the strain is tensile (and are therefore absent in compressively strained Si1?xGex), are more effective at impeding dislocation glide and it is their increased density in thicker layers which yield enhanced stability, even after high temperature annealing.