The effect of cooling rate on the defect structure in Si(1-x)G(x), layers grown on Si (100) substrates has been studied. SiGe layers 200 nm thick. with germanium fractions between 0.09 and 0.14 and capped with a 50-nm layer of Si, were grown by molecular beam epitaxy at temperatures of 550, 710 and 800 degrees C. The defect structure in the layers was studied by means of high resolution double crystal X-ray diffractometry and transmission electron microscopy for two different modes of cool-down. In the 'fast' mode, the heat source was turned off abruptly, while in the 'slow' mode the power was lowered ill small steps equal to about 2% of the initial power, The system was then allowed to reach equilibrium before the power was lowered again. X-ray rocking curves showed significant broadening after rapid cool-down from higher temperatures, whereas the corresponding strains decreased only slightly and independently of the rate of cool-down. In ol der to explain these results, a model is proposed in which the buried layer is distorted into small, slightly misoriented regions by rapid cooling, Thus, the original strain remains unchanged within the small regions and the X-ray profile is broadened primarily by crystalline distortion and without a significant contribution from dislocations. The distortion can lend to fracture for rapid cool-down from sufficiently high temperatures, e.g., 800 degrees C. The model is consistent with both transmission electron microscopy and calculations of stress due to non-uniform cooling.