The oxygen permeability of CaAl0.5Fe0.5O2.5+delta brownmillerite membranes at 1123-1273 K was found to be limited by the bulk ionic conduction, with an activation energy of 170 kJ/mol. The ion transference numbers in air are in the range 2 x 10(-3) to 5 x 10(-3). The analysis of structural parameters showed that the ionic transport in the CaAl0.5Fe0.5O2.5+delta lattice is essentially along the c axis. The largest ion-migration channels are found in the perovskite-type layers formed by iron-oxygen octahedra, though diffusion in tetrahedral layers of the brownmillerite structure is also possible. Heating up to 700-800 K in air leads to losses of hyperstoichiometric oxygen, accompanied with a drastic expansion and, probably, partial disordering of the CaAl0.5Fe0.5O2.5+delta lattice. The average thermal expansion coefficients of CaAl0.5Fe0.5O2.5+delta ceramics in air are 16.7 x 10(-6) and 12.6 x 10(-6) K-1 at 370-850 and 930-1300 K, respectively. (C) 2003 Elsevier Science Ltd. All rights reserved.