The interface between thin films of the YBa/sub 2/Cu/sub 7-x/ superconductor and metallic and insulating overlayers is studied using films prepared in situ by coevaporation employing ozone vapor oxidation. Thin layers of Y, Y/sub 2/O/sub 3/, and BaF/sub 2/ can be evaporated at various stages of the YBa/sub 2/Cu/sub 7-x/ cool-down process. The effect of the overlayers on the oxidation state of the superconductor copper signal can be assessed using X-ray photoelectron spectroscopy (XPS). It is shown that ultrathin Y overlayers are rendered inert with respect to an underlying YBa/sub 2/Cu/sub 7-x/ film surface by the application of a pure ozone flux at the surface during deposition. This is accomplished at ambient substrate temperatures and at the highest temperature used in the processing of the superconductor. The preparation of epitaxial or highly oriented Y/sub 2/O/sub 3/ barriers is thus possible without an apparent effect on the integrity of the high-T/sub c/ surface and. perhaps, the superconducting order parameter. The barrier can be applied at a high substrate temperature where the mobility of the evaporant is large and crystallinity and smoothness can be expected to be optimum. It is expected that these results applied to high-quality a-axis oriented films may allow the development of improved-quality planar high-T/sub c/ tunnel junctions.