High resolution x-ray diffraction in combination with kinematical scattering simulations based on finite element calculations for the elastic strain field has been utilized to investigate the chemical composition of SiGe/Si(001) Stranski-Krastanov islands. Therefore the intensity distribution in the vicinity of the symmetric (004) reflection has been three-dimensionally measured and compared with respective simulations. Around an asymmetrical reflection, e.g. Si(113), the impact of lateral and vertical strain tensor components on the diffraction pattern decouples. This allows to distinguish between tetragonally distorted regions at the island bottom and elastically relaxed unit cells of cubic shape at the island apex in accordance with finite element calculations. Artificially switching off and on the various scattering abilities of silicon and germanium in otherwise equivalent simulations yields practically no difference in the resulting scattering. Thus, at x-ray energies far from absorption edges, the diffuse scattering is dominated by elastic strain which is caused by the chemical profile.