Oxygen-ionic and electronic transport in dense (SrFe)(1-x)(SrAl2)(x)O-z composites, consisting of strontium-deficient Sr(Fe,Al)O3-delta and SrAl2O4 phases, is determined by the properties of perovskite-like solid solution. Increasing the content of SrAl2O4, with a total conductivity as low as 5 x 10(-7) - 10(-5) S x cm(-1) at 973-1273 K in air, results in the gradual decrease of the partial conductivities, but also enables the suppression of thermal expansion. Compared to single-phase SrFe1-xAlxO3-delta, (SrFe)(1-x)(SrAl2)(x)O-z composites exhibit enhanced thermomechanical properties, while the oxygen permeability of these materials has similar values. The composite membranes exhibit stable performance under air/(H-2-H2O-N-2) and air/(CH4-He) gradients at 973-1173 K. The oxidation of dry methane by oxygen permeating through (SrFe)(0.7)(SrAl2)(0.3)O-z results in dominant total oxidation, suggesting the necessity to incorporate a reforming catalyst into the ceramic reactors for natural gas conversion. (c) 2006 Elsevier B.V. All rights reserved.