Epitaxial growth of GeSi (001) in the Stranski-Krastanow regime results in the formation of island ensembles with various sizes and morphologies. During formation there is generally a strain-driven Si diffusion into the Ge islands. We investigate this issue in an epilayer grown by molecular-beam epitaxy containing pyramids, domes, and superdomes. A series of samples obtained by wet chemical etching of the original layer for different times in diluted hydrogen peroxide was evaluated by atomic force microscopy, spectroscopic ellipsometry, and Raman scattering. The average island composition as etching proceeds becomes Si richer, changing from about Si0.2 Ge0.8 to Si0.35 Ge0.65, whereas the lattice strain increases, in particular, the material at the island summits is essentially relaxed. The composition of the wetting layer is nearly Si0.45 Ge0.55. The results also reveal relatively Si-rich nuclei of a uniform size of ≃100 nm for all domes and superdomes, in accordance with a dislocation-induced growth mechanism of superdomes. © 2005 American Institute of Physics.