Abstract: A composition comprising an electron-conducting component and an oxide ion-conducting component, characterised in that the electron-conducting component is also an oxide ion-conductor, the composition being suitable for use in a selective oxygen permeable membrane for separating oxygen from a gaseous mixture comprising oxygen, such as air, the separation optionally being carried out in a reactor comprising a first and second zone, in which an oxygen-containing gas is fed to the first zone, and a reactant fed to the second zone, in which an oxygen consuming reaction occurs in the second zone of the reactor.