The composition of the oxides formed at room temperature in the atmosphere or by rf oxidation on the surfaces of NbN thin films were characterized by means of X-ray photoelectron spectroscopy. The experimental Nb3d spectra show that the interfacial phase formed between NbN substrate and the niobium pentoxide is different from the case of pure niobium. It is Nb2O3 but not a mixture of NbO and NbO2. The surface of NbN seems to have the ability of protecting itself from the formation of conducting suboxides such as Nb2O, NbO etc. This is useful for improving the quality of the superconducting tunnel junction with oxide barrier.