Dense ceramic membranes made from mixed conductors are interesting because of their potential applications for methane conversion into syngas (H-2 and CO mixture). Such membranes need to present a low differential dimensional variation between the opposite faces submitted to a large gradient of oxygen partial pressure, in order to minimize mechanical stresses generated through the membrane thickness. Besides, high oxygen permeability is required for high methane reforming rate. La(1-x)SrxFe(1-y)GayO3-delta materials fulfil these two main requirements and were retained as membranes in catalytic membrane reactors (CMR). The variations of expansion and oxygen permeation of La(1-x)SrxFe(1-y)GayO3-delta perovskite materials with the partial substitution of lanthanum and iron cations, temperature and oxygen partial pressure, were studied. For low temperatures (<800 degrees C), the thermal expansion coefficient (TEC) value of La(1-x)SrxFe(1-y)GayO3-delta materials is independent of cation substitution and of oxygen partial pressure in the range tested (10(-5) to 0.21 atm). For higher temperatures (>800 degrees C), TEC, then dimensional stability of the membrane, and oxygen permeation of La(1-x)SrxFe(1-y)GayO3-delta materials, are significantly affected by Sr content and oxygen partial pressure. Ga has a stabilisation effect on the TEC and has no influence on oxygen permeation flux. A good compromise between dimensional stability and oxygen permeation of materials was found to be La0.7Sr0.3Fe0.7Ga0.3O3-delta composition. (C) 2008 Elsevier B.V. All rights reserved.