Percolative and non-percolative fluorite type Bi1.5Y0.3Sm0.2O3 (BYS) and silver dual-phase membranes were prepared by mixing BYS and Ag powders. Oxygen permeation and electric conduction through these BYS/Ag dual-phase membranes were studied in thr temperature range of 650-850 degrees C. The percolative BYS/Ag 40% and non-percolative BYS/Ag 30% membranes exhibit about 50-fold and 10-fold increase, respectively, in oxygen permeation flux compared to BYS membranes. For a 1.3 mm thick BYS/Ag 40% membrane, an oxygen permeation flux of 5.8 x 10(-7) mol/cm(2) s was measured at 850 degrees C with oxygen partial pressures at the feed and permeate sides being 0.21 and 0.009 arm, respectively. The activation energies for oxygen permeation in the temperature range of 650-850"C are 79.30 and 87.30 kT/mol for the BYS/Ag 30% and BYS/Ag 40% membranes, respectively. For the non-percolative BYS/Ag membrane, the presence of Ag enhances the surface reaction rate for oxygen permeation. The continuous silver phase in the percolative BYS/Ag membrane increases both the electronic conductivity and surface reaction rate which result in substantial improvement of oxygen permeance.