We have developed a new method to analyze the stress state of heteroepitaxial systems using X-ray multiple diffraction (MD). A fitting program extends the MD theory for mosaic crystals to provide the position and profile of the normal and hybrid MD peaks. We use surface secondary beams in order to achieve high resolution in intensity and peak position. These conditions together with the absorption involved in the LS hybrid reflections enable us to test the stress state of the layer by determining the misfit dislocation and the degree of cohesion between the buffer and epitaxial layers. Here, the method was applied in the analysis of thin (500 Angstrom) and thick (1.2 mu m) GaAs layers grown by Vacuum Chemical Epitaxy (VCE) on Si (001).886465469