Abstract: Method for producing single crystals of YBa.sub.2 Cu.sub.3 O.sub.7 which undergo a transition to the superconducting state above 90K, comprising:(a) preparing a flux composition by mixing BaO.sub.2 and CuO powders in the molecular weight ratio of approximately 1:1, layering on top of the mixture powdered YBa.sub.2 Cu.sub.3 O.sub.7 in an amount up to approximately 20% of the total weight of the BaO.sub.2 --CuO mixture, firing the mixture in air at approximately 930.degree. C.-950.degree. C. for approximately 24 hours, and thereafter raising the temperature from 930.degree. C.-950.degree. C. to approximately 1025.degree.-1030.degree. C. within 1/2 hour and maintaining the temperature at approximately 1025.degree. C.-1030.degree. C. for approximately one hour to one and one-half hours;(b) cooling thereafter the flux to approximately 1020.degree. C.-980.degree.;(c) providing an oxygen gas flow to the flux when the temperature is approximately 1020.degree. C.-980.degree. C.