The adsorption of three monolayers (3 MLs) of Sb at - 120 degrees C on the Si(001)c(4 x 2) surface and the subsequent annealing up to 650 degrees C were followed by high resolution core level spectroscopy and RHEED analysis. By relating the electron diffraction patterns to the Si2p and Sb4d core level spectra measured after each annealing cycle, it was possible to monitor the evolution of the Sb/Si interface structure up to the achievement of a diffuse (2 x 1) reconstruction. Even after the annealing to 650 degrees C, the quality of the surface reconstruction remained poor, as attested by the RHEED pattern, by the intensity of the Si2p surface component and by the broadening of the Sb4d lineshape.