This study examined the effect of ion irradiation and subsequent thermal annealing on GeSi/Si strained-layer heterostructures. Comparison between samples irradiated at 253°C with low energy (23 keV) and high energy (1.0 MeV) Si ions showed that damage within the alloy layer increases the strain whereas irradiation through the layer/substrate interface decreases the strain. Loop-like defects formed at the GeSi/Si interface during high energy irradiation and interacting segments of these defects were shown to have edge character with Burgers vector a/2<110>. These defects are believed responsible for the observed strain relief. Irradiation was also shown to affect strain relaxation kinetics and defect morphologies during subsequent thermal annealing. For example, after annealing to 900°C, un-irradiated material contained thermally-induced misfit dislocations, while ion-irradiated samples showed no such dislocations.