(Ba0.5Sr0.5)(Co0.8Fe0.2)(y)O3-delta ((BS)(CF)(y), 1.00 >= y >= 0.77) oxides were investigated for oxygen separation application with emphasis on long-term operational stability. Pure phase cubic perovskite was formed at y >= 0.83. The oxygen nonstoichiometry increased while electrical conductivity and permeability decreased with the decrease of y. However, the (BS)(CF)(0.97) membrane still displayed an attractive oxygen flux as high as 2.4 x 10(-6) mol cm(-2) s(-1) at 900 degrees C, as compared to 2.5 x 10(-6) mol cm(-2) s(-1) for a cation stoichiometric BSCF membrane. The B-site deficiency greatly restrained the A-site cation diffusion and stabilized the perovskite structure and permeation properties of the membranes. During the long-term operation of the (BS)(CF)(0.97) membrane at 850 degrees C for more than 300 h, a stable permeation flux of (1.8 +/- 0.3) x 10(-6) mol cm(-2) s(-1) was achieved. Further investigation demonstrated that the improved importance of oxygen bulk-diffusion regime in the rate-determination of oxygen permeation through the membranes. (C) 2008 Elsevier B.V. All rights reserved.