He ion channeling and backscattering experiments have been performed on MBE-grown Si1−xGex/Si strained layers to investigate the tetragonal lattice distortion and the Ge diffusion after thermal annealing. The tetragonal strain of the epitaxial layers has been determined by measuring the angular deviations of inclined 〈110〉 orientations of the adjacent layers. The measured angular shifts indicate a nearly pseudomorphic layer growth of the virgin sample. The angular yield scans of the Ge signal show a pronounced asymmetry, caused by beam steering in the tetragonally distorted lattices of the different layers. Thermal annealing above 800 °C results in strain relaxation and in layer mixing due to the diffusion of germanium. The observed Ge diffusion is strongly enhanced by the elastic strain in the layered structure.