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(La,Sr)FeO3 mixed conducting perovskites are considered as interesting candidates for oxygen separation membranes but they suffer from limited structural stability in a large oxygen partial pressure (pO2) gradient, because of their propensity for chemical expansion. Partial substitution of Fe with more stable elements tends to improve the stability while penalizing the electronic and ionic conductivities. In this study, we investigate the effect of 10% Ta substitution on the oxygen transport properties and stability of La0.5Sr0.5FeO3. For this purpose, the material was evaluated as a membrane in a CPOX reactor. The oxygen permeation through a ~ 3 cm2 pellet sample was first measured under air/Ar gradient in the temperature range of 800 to 1000 °C. The measured flux was 0.1 µmol cm− 2 s− 1 at 900 °C, which was a factor of 2 lower than for the Ta-free material. Methane was then introduced into the system and reacted in a catalytic bed with oxygen that has permeated through the membrane to form syngas (H2, CO). As a result, the oxygen flux increased by a factor of 9, reaching 0.9 µmol cm− 2 s− 1 at 900 °C. The reactor was operated at 1000 °C for another 1000 h. During this time, the oxygen permeation flux decayed by ca. 4%/1000 h. The test was stopped after more than 2000 h of operation and the membrane analyzed by electron microscopy.