The critical role of gate oxide in ultra-scaled devices is being investigated in terms of the properties of rare earth oxides as high dielectric constant (high-κ) materials to replace SiO2. In particular, the combination of rare earth oxides with high-mobility substrates, like Ge and GaAs, could offer the possibility to improve the interface properties. Among the different properties under investigation, the band alignment at the interface is a key issue because it affects the tunneling behavior of a device. Internal photoemission and X-ray photoelectron spectroscopy are useful techniques to experimentally determine the band offset at the semiconductor/oxide interface. After a detailed description of these two methods, we present a review of the data available in the literature on the interface of different high-κ oxides on silicon. Finally, we report our measurements of the Lu2O3 band alignment on various semiconductor substrates. A conduction band offset value of 2.1 eV has been obtained by internal photoemission for Lu2O3 films grown on Si, Ge, and GaAs. X-ray photoelectron spectroscopy measurements of the valence band offset were performed on Ge/Lu2O3 heterojunction. The results are in excellent agreement with those obtained using internal photoemission.