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 Si1-x Tix 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.