Strain relaxation and consequent generation of defects in GaAs/InxGa1-xAs/GaAs heterostructures with x=0.05–0.3 prepared by metalorganic chemical vapor deposition have been studied using photoluminescence, positron beam, and x-ray diffraction techniques. Photoluminescence studies have indicated peak shifts and broadening in the spectra as the In concentration is increased. Broadening is attributed to defect generation, caused by In substitution beyond the critical limit. Depth resolved defect-sensitive S-parameter measurements, using a low energy positron beam, exhibit an increase in the S parameter over a depth range corresponding to the In substituted layers as the In concentration is increased. The results are suggestive of the production of open volume defects like misfit dislocations in the In substituted layer. A simplified analysis of positron beam data shows that a 5% In sample is defect-free, indicating that it is pseudomorphic to the substrate. A 10% In sample is in the transition region, while higher In concentration samples indicate a large concentration of defects. X-ray diffraction studies have revealed the in-plane and out-of-plane strains in the samples and it is found that increased In substitution leads to larger strains and an increased degree of strain relaxation. Using the experimentally determined strain parameters, a dynamical theory based simulation of x-ray profiles has been made for comparison with the measured profiles. The combined study of the three techniques clearly establishes the interplay between the In substitution and strain relaxation leading to defect generation in the InGaAs system. © 2000 American Institute of Physics.