We report on synchrotron radiation photoelectron spectroscopy studies of the Si 2p core levels obtained with very high resolution from Sb overlayers onto the low-index silicon surfaces Si(110), Si(100) and Si(111). The improved energy resolution of the third generation synchrotron radiation facilities has shown the presence of a strong interfacial component shifted 0.24, 0.2 and 0.13 eV, respectively, on the high binding energy side respect to the bulk peak. Such Si-Sb interface contribution is ascribed to charge transfer between the metal and the topmost silicon atoms and to the latest layers of silicon atoms distorted by the reconstruction from the ideal bulk position. A small component is always present on the high kinetic energy side and is presumably due to contribution from defects.