Abstract: The present invention relates to a dense single-phase membrane having both high ionic and electronic conductivity and capable of separating oxygen from an oxygen containing gaseous mixture, where the membrane comprises a mixed metal oxide material with interstitial oxygen excess represented by the formula AyA′y′A″y″BxB′x′B″x″B′″x′″O4+&dgr;, where A, A′ and A″ are chosen from group 1, 2 and 3 and the lanthanides; and B, B′, B″ and B′″ are chosen from the transition metals according to the periodic table of the elements adopted by IUPAC wherein 0≦y≦2, 0≦y′≦2, 0≦y″≦2, 0≦x≦1, 0≦x′≦1, 0≦x″≦1, 0≦x′″≦1, and x and y each represents a number such that y+y′+y″=2, x+x′+x″+x′″=1, and &dgr; is a number where 0≦&dgr;<1 quantifying the oxygen excess.