The Ge L-3 edge x-ray absorption spectra of a series of Ge/Si heterostructures grown on Si(001) substrates were investigated using linearly polarized synchrotron radiation. By making use of a multiple scattering approach we reproduced the experimental spectra for different structural models and several degrees of strain and intermixing. Evidences of interfacial intermixing processes were found even at room temperature. Such an intermixing is strongly inhibited by Sb assisted growth.