The crystalline structures of metamorphic III-V heterostructures, GaSb/(001)GaAs and GaAs/(001)InP, have been studied by image analyses of high resolution electron microscopy (HREM) micrographs. Digital processing has been performed in order to evaluate the extension of the distorted zone between the substrate and the fully relaxed epilayer. GaSb grown by MBE at 470 degrees C on GaAs is known to be perfectly relaxed by a Lomer dislocation network directly created during the island growth. The transition zone is measured to be few atomic planes thick. For the GaAs, grown on InP by MBE at 450 degrees C, the thickness of the transition zone observed at the interface is IO nm. The misfit dislocation network involved in the relaxation process appears to be poorly organized with mainly partial and 60 degrees dislocations.