Lattice distortions in three-dimensional coherent In0.35Ga0.65As islands grown by molecular beam epitaxy at 510 °C on GaAs have been imaged by high resolution electron microscopy. The strain fields are determined from the corresponding digital images, either by direct measurement of the lattice distortions or by combining real space and Fourier space information, with an uncertainty ΔЄ=2×10-3. The strain fields are also simulated through finite-element calculations, taking into account the strain relaxation due to the low thickness of the electron-transparent specimens. The significant differences found between experimental and calculated strain fields are attributed to In segregation within the islands. Bidimensional compositional maps are then established showing that the In concentration in the central part of the islands (up to ∼50%) is significantly higher than the nominal concentration (35%), whereas it is lower (down to ∼20%) at the edges of the islands. © 1999 American Institute of Physics.