Moderate additions of monoclinic SrAl2O4 to perovskite-type SrFe(Al)O3-delta mixed conductors improve the sinterability and thermomechanical properties, including the thermal shock resistance, Vickers hardness, and fracture toughness, and decrease thermal expansion. The iron solubility in SrAl2O4, a mixed ionic and n-type electronic conductor with insulating properties, is lower than 5%. The total conductivity of SrAl2O4 ceramics in air varies in the range 10(-7) - 10(-5) S/ cm at 973 - 1223 K. The transport properties and phase stability of dual-phase (SrFe)(1-x)(SrAl2)(x)O-z (x = 0.3 - 0.7) composite membranes, where the partial dissolution of strontium aluminate in the ferrite phase leads to formation of A-site-deficient Sr1-yFe1-2yAl2yO3-delta (y approximate to 0.08 - 0.12), are determined by the perovskite component. The total conductivity and Seebeck coefficient oxygen partial pressure dependencies exhibit general trends typical for SrFeO3-based solid solutions. Although the conductivity and oxygen permeability of (SrFe)(1-x)(SrAl2)(x)O-z composites decrease with increasing x, the permeation fluxes through (SrFe)(0.7)(SrAl2)(0.3)O-z ceramics are comparable to those through single-phase SrFe0.7Al0.3O3-delta. Under high p(O2) gradients such as air/(H-2 - H2O), the oxygen transport is limited by surface-related processes, enabling stable operation of (SrFe)(0.7)(SrAl2)(0.3)O-z membranes. This composition was selected for fabrication of tubular membranes by the cold isostatic pressing. Surface modification of (SrFe)(0.7)(SrAl2)(0.3)O-z in order to enhance the exchange kinetics was found inappropriate from a stability point of view. (C) 2006 The Electrochemical Society.