Praseodymium sequioxide (Pr 2 O 3 ) is among the most promising hetero-oxides which are treated as candidates of choice to replace SiO 2 as the gate dielectric material for sub-0.1mm CMOS technology. However, in order to enable the process integration the hetero-oxides require substantial characterization. In particular, the basic interaction mechanism at the interface to the silicon substrate is a key issue. A good knowledge of this mechanism is required to address the reliability issues. The challenges in material science are to understand the chemical bonding of the heterooxides to silicon in a microscopic scale. This report focuses on the studies of Pr 2 O 3 ultra thin (o1 nm) epitaxial layers prepared in situ. Photoelectron spectroscopy (PES) is used for spectroscopic characterization of the interface elements as well as for a nondestructive depth profiling. We report on the atomic origin of the valence and conduction band states as revealed by resonant PES, on the valence band discontinuity, on the formation of interface dipole moment, and on the stability regime of a mixed silicate phase. Within the latter, the chemical state of Si atoms at the interface is identified and the chemical stability of the various oxide phases is discussed.