Ceramics with mixed ionic and electronic conductivity are being investigated for oxygen separation from air, intended for the production of syngas. Asymmetric membranes, consisting of a dense membrane co-sintered with a porous support are expected to achieve a high flux of oxygen and at the same time adequate strength. As the porous substrate is mainly intended for strength contribution, it is crucial that the flux of oxygen through the dense membrane is not limited by the flow of air through the porous substrate. An oxygen flux of 10 ml min-1 cm-2 should be achieved at operating temperatures (800-1000 °C) for the substrate to be commercially attractive. A biaxial strength of 34MPa has been obtained by similar porous substrates.Four different strategies for achieving high permeating substrates made by solid state La0.2Sr0.8Fe0.8Ta0.2O3-x have been evaluated. Pressed porous substrates were produced both with and without the use of pore formers. The most promising compositions with respect to porosity and permeability were tape casted and characterized with respect to porosity, permeability and strength.