At high strontium doping levels, perovskite oxides containing iron have suitable stability and transport properties for use as oxide ion transport membranes. In our studies of these materials, we have investigated the pO(2) and temperature dependence of the conductivity and non-stoichiometry of La1-xSrxFe1-yMyO3-delta (M=Cr, Ti) by using electrochemical cells and the thermal expansion by dilatometry. Non-equilibrium behavior is observed in both the chemical expansion data and also in the conductivity and stoichiometry and suggests the occurrence of microscopic phase segregation on reduction. Analysis of the microstructure of quenched samples confirms the occurrence of local phase separation. Bulk diffusion and surface exchange coefficients under near-gradientless conditions have been determined by the electrical conductivity relaxation (ECR) technique and by isotope exchange depth profiling (IEDP). Evaluation of transport under a chemical gradient was accomplished by transient isotopic tracing of operating membranes. The isotope transients (O-16(2)-O-18(2)) were performed on tubular membranes operating at steady state at temperatures between 1023 K and 1173 K and allow an unambiguous separation of surface and bulk resistances to oxygen permeation under steady state conditions, a separation not possible by permeation measurements alone. (c) 2006 Elsevier B.V. All rights reserved.