A method is described of producing a composite oxygen ion membrane and a composite oxygen ion membrane in which a porous fuel oxidation layer and a dense separation layer and optionally, a porous surface exchange layer are formed on a porous support from mixtures of (Ln1-xAx)wCr1-yByO3-δ and a doped zirconia. Preferred materials are (La0.8Sr0.2)0.95Cr0.7Fe0.3O3-δ for the porous fuel oxidation layer, (La0.8Sr0.2)0.95Cr0.5Fe0.5O3-δ for the dense separation layer, and (La0.8Sr0.2)0.95Cr0.3Fe0.7O3-δ for the porous surface exchange layer. Firing the said fuel activation and separation layers in nitrogen atmosphere unexpectedly allows the separation layer to sinter into a fully densified mass.