Thin strained Si layers grown on SiGe layers graded to 20% Ge were studied for resistance to relaxation. It was observed that in the presence of ∼105/cm2 threading dislocations from the underlying graded layers, the barrier to misfit dislocation formation is sufficiently reduced to induce relaxation in Si layers even when the layer thickness is less than the predicted critical thickness. Raman spectroscopy revealed that elastic strain accumulation in the uniform SiGe layers is a significant contributor to strain relaxation in the Si cap layers. Upon annealing, thermal mismatch causes the Si layers to relax further, but most of the strain relaxation is accommodated by elastic strain increase in the SiGe layers. This prevents the rampant increase in defect density that would otherwise accompany the strain relaxation. Annealing in an oxidizing ambient appears to pin pre-existing threading dislocations causing nucleation of new threading dislocations and short misfit segments to relieve the thermal mismatch stresses. © 1999 American Vacuum Society.