Chemical composition of very thin niobium oxide layers grown on the surface of 200-nm niobium films was determined by X-ray photoelectron spectroscopy (XPS or ESCA). The oxides were fabricated by rf plasma oxidation under conditions that minimize the incorporation of impurities such as carbon in the Nb2O5 Josephson junction tunnel barriers. Nb 3d XPS core level peak positions, areas, and area ratios were used to determine the presence of different oxidation states of Nb, their relative abundance, and surface oxide thickness. The observed composition deviated from what is usually assumed for an ideal model superconductor-insulator structure. The oxide was incompletely oxidized Nb2O5-δ with δ decreasing from 0.2 to 0.1 with increasing oxide thickness from 2 to 4 nm. A 0.5-nm interface region was observed between the metal and oxide. © 1983 Plenum Publishing Corporation.