X-ray photoelectron spectroscopy (XPS) was used to evaluate the interfaces between ion-beam sputter deposited SiO² and TiO² layers, their free surface, and their interface with a silicon substrate. XPS depth profiles of nominally abrupt SiO²/TiO interfaces showed mixing over an 7 nm range, but no valence states other than Si⁺⁴ and Ti⁺⁴. Thus the interfacial region consists of Si^1-x Ti^x 0² where x varies from 0 to 1. The free surface of either SiO² or TiO² extends over 1.5 nm and shows evidence of OH bonds, presumably due to absorbed water. Interfaces between SiO², or TiO², and an optically smooth silicon substrate are about 3 nm wide and exhibit +2 and 0 bonding states for Si, and +2 for Ti, states characteristic of oxygen deficient films. The TiO²/Si interface also contains Ti-Si bonds, or partial silicide formation.