The high temperature crystal chemistry and oxygen permeation properties of the cation-ordered LnBaCo(2)O(5+delta) perovskite oxides [lanthanide (Ln) = Pr, Nd, and Sm] have been investigated in comparison with the cation-disordered La0.5Ba0.5CoO3-delta perovskite. The LnBaCo(2)O(5+delta) (Ln = Pr, Nd, and Sm) oxides exhibit a metal-insulator transition at T < 200 degrees C, as evidenced by total conductivity measurements and high temperature X-ray diffraction data and an oxygen vacancy order-disorder transition at T > 350 degrees C in air, as evidenced by an orthorhombic to tetragonal transition. At a given temperature, the oxygen permeation flux decreases from Ln = La to Nd to Sm due to the changes in crystal symmetry and lattice strain. The oxygen permeation mechanism in the Ln = Nd is bulk-diffusion-limited rather than surface-exchange-limited for membrane thickness L >= 1.1 mm. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3231501] All rights reserved.