The project addresses the problem of chemical and mechanical stability of dense ceramic oxygen separation membrane for the partial oxidation (POX) of methane to synthesis gas. In operating conditions, the membranes are exposed to strong thermal and composition gradients which result in local stresses and possible fracture. A series of candidate materials were synthesised and characterised. Samples were prepared in the form of pellets, rods and tubes for the measurement of their properties (thermal and chemical expansion, mechanical properties and oxygen permeation flux). Based on the results, modelling was performed: an optimal operational regime for the POX-reactor was first determined at a system-level. By preheating the fuel up to 900° the temperature gradient along the reactor could be reduced to a sustainable level. Detailed 2D (axi-symmetrical) numerical modelling of a membrane tube showed that the thermal gradients were mainly axial due to the heat transfer limitations at its surfaces. On the other hand, chemical gradients were mainly radial and also strongly dependent on surface exchange limitations. The stess functions resulting from radial strain were calculated for three different geometries: a) self-supported tube, dense coating on the external (b) and internal (c) surface of a porous support. The optimal configuration could be determined (air flowing to inside). A key materials fabrication focus is on chemical stabilisation of the compounds under the harsch operating conditions. The base material La0.5Sr0.5FeO3 was chosen, with Ti-stabilisation (10%, 20%) on the Fe-site, amongst a vast oxploration series of other B-site substitutions. Isothermal expansion was thus reduced from 0.45% to 0.2%, however, at the expense of a diminished oxygen permeability (-40% at 1000°C).