The depth profile of high-resolution photoelectron spectra at Ga2O3(Gd2O3)-GaN and Gd2O3-GaN interfaces grown by molecular beam epitaxy (MBE) has been obtained by using synchrotron radiation beam and low-energy Ar+ sputtering. The data indicate that both of the oxide layers are highly hygroscopic with the observation of O-H bonding. The valence-band offsets (&UDelta; Ev) of ∼ 1.1 and 1.0eV were also measured for the Ga2O3(Gd2O3)-GaN and Gd2O3-GaN interfaces, respectively. © 2005 Elsevier B.V. All rights reserved.