NOVELTY - A gas-impermeable mixed phase ceramic catalytic reactor membrane, comprises 80 weight% (wt.%) or more mixed ionic and electronic conducting phase (MIEC) and 0.1-20 wt.% of one or more structurally distinct second phases which impart mechanical strength to the membrane.    USE - For hydrocarbon processing, separation of oxygen from oxygen-containing gas (e.g. air) and production of pure oxygen.    ADVANTAGE - The membrane has excellent ionic and electronic conductivities, and has improved mechanical strength particularly under catalytic membrane reactor operating conditions. The membrane particularly used for long-term reactor operation. The MIEC membranes behaves as short-circuited electrochemical cells, with appropriate catalysts applied for promoting each half reaction on their respective oxidizing and reducing surfaces.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a ceramic membrane reactor for production of synthesis gas, which has gas-impermeable membrane;    (2) a method of oxidizing a reactant gas capable of reacting with oxygen, which involves:    (a) providing a catalytic membrane reactor cell comprising an oxidation zone and reduction zone separated by gas-impermeable mixed phase ceramic membrane having reduction surface and an oxidation surface,    (b) heating reactor cell to 300-1200 degreesC,    (c) passing an oxygen containing gas in contact with the reduction surface of membrane of the heated reactor in the reduction zone, and    (d) passing reactant gas in contact with the oxidation surface of membrane of heated reactor in oxidation zone to oxidize the reactant gas;    (3) production of synthesis gas by reaction of oxygen containing gas with the hydrocarbon, which is same as that of oxidation of reactant gas (ii);    (4) formation of mixed phase ceramic material which involves:    (a) combining metal precursors of the metal of the MIEC metal oxide to obtain a combination of metal ions in which the relative amounts of metal ions are off-stoichiometric for at least one metal ion with respect to the formula of the MIEC metal oxide,    (b) milling the combined metal precursors to obtain a homogeneous mixture,    (c) calcining the homogeneous mixture to obtain the MIEC metal oxide in combination with a second phase containing at least one metal ion of the metal ions of the MIEC to form a homogeneous powder mix,    (d) reducing the particle size of the powder mix, if necessary, to less than 2 microns, and    (e) repeating the steps C and D until the reaction is completed.