Abstract: The superconducting material YBa.sub.2 Cu.sub.3 O.sub.7, which loses its superconducting qualities when exposed to water, is passivated against such water damage by a thin film on its surface comprised of a mixed phase which includes YBa.sub.2 Cu.sub.3 O.sub.(7-x) F.sub.2x, where 0<X.ltoreq.1/2.