The composition and chemical bonding of the first atoms across the interface between Si(001) and the dielectric determine the quality of dielectric gate stacks. An analysis of that hidden interface is a challenge as it requires both, high sensitivity and elemental and chemical state information. We used SR based photoelectron spectroscopies and, in particular, X-ray absorption spectroscopy in total electron yield and total fluorescence yield at the Si2p and the O1s edges to address that issue. We report on results of Hf-oxide prepared by ALD and compare to Pr2O3 / Si(001), and compare the two to the SiO2 / Si(001) system as a reference. For both, Hf-oxide and Pr-oxide thin films we find evidence for the silicate formation at the interface as derived from the characteristic features at the Si2p and the O1s edges.