Powders of Bi1.5Y0.5-yCuyO3-delta (BYC) and Bi1.5Y0.3Sm0.2O3 in the fluorite phase structure were prepared by the citrate method. The chemical reactions involved in the synthesis of these powders were proposed and verified experimentally. Dense BYC and BYS membranes were prepared from the powders by the press and sintering method. The optimum conditions for powder preparation as well as for membrane fabrication were determined in this study with the aid of various characterization methods. The BYC and BYS membranes were gas-tight to helium. These doped bismuth-yttrium (BY) oxide membranes show similar ionic conductivity but much higher oxygen permeability (electronic conductivity) as compared to the undoped BY membrane.