NOVELTY - Process for preparing a supported, ceramic, tubular membrane (A) comprising two coaxial layers, along an axis, i.e. a layer of support material (S), of thickness es, and a layer of activer material, of thickness em, where em is smaller than es.    USE - (A) is used (i) to produce catalytic membranes, by application of a reforming catalyst to the external surface of the active material layer; (ii) to prepare ceramic oxygen generators (claimed) and (iii) for separation and/or preparation of hydrogen from gas mixtures. A particular application is use in solid-state electrochemical reactors, e.g. reforming methane to synthesis gas.    ADVANTAGE - Unlike the known methods for making (A), this process allows the ratio between thicknesses of the two layers to be varied, over two orders of magnitude.    DETAILED DESCRIPTION - Process for preparing a supported, ceramic, tubular membrane (A) comprising two coaxial layers, along an axis, i.e. a layer of support material (S), of thickness es, and a layer of activer material, of thickness em, where em is smaller than es. It comprises:    (A) forming (A) by simultaneous coextrusion of a paste (Ps) of S and a paste (Pm) of M, at equal flow rates along the axis;    (B) drying the coextrudate;    (C) removing binder; and    (D) co-calcining by heat treatment.    INDEPENDENT CLAIMS are also included for the following:    (1) device for performing step (a) of the new process;    (2) method for preparing a catalytic membrane reactor by applying a reforming catalyst to the external face of M in (A); and    (3) method for preparing a ceramic oxygen generator or a solid fuel cell in which (A) is prepared directly by the new process.    INORGANIC CHEMISTRY - Preferred Process: The initial coextrudate may be cut into tubular pieces of identical length. Preferred Pastes: Ps typically contains, by volume, 28-50% powdered (S), or powdered materials that form S during the process; 15-40% pyrolyzable pore-former; 0.5-5% dispersant; 1-15% organic binder; 0-5% plasticizer; 1-15% lubricant and 10-50% water. Pm typically comprises, by volume, 40-70% powdered M, or powdered materials that form M during the process; 0.5-8% dispersant; 1-15% organic binder; 0-5% plasticizer; 1-15% lubricant and 15-50% water. Preferred Materials: M contains, by volume, 75-100, especially at least 95, % conductors (C1) of electrons and oxygen anions, i.e. doped ceramic oxide; 0-25, especially up to 5, % ceramic, metal or alloy (C2) and 0-2.5, preferably up to 0.5, % compounds formed by reaction between C1 and C2. Most preferably C1 is 100% and a preferred material is La(1-x)SrxFe(1-v)Mb''v O3~d. S is especially an oxide (e.g. of calcium, aluminum or titanium), a mixed metal oxide, silicate, perovskite, carbide or nitride.    x = 0-0.5;    v = 0-0.9;    delta = number that ensures electrical neutrality;    Mb'' = transition or alkaline earth metal, aluminum, indium, gallium, germanium, antimony, bismuth, tin, lead or titanium    . INSTRUMENTATION AND TESTING - Preferred Device: The device for step (a) comprises (i) a screw die for coextrusion, in which flows of the two pastes are kept separate; and (ii) extruders for each paste, comprising a double-walled body; a cylindrical extension for recording temperature and pressure; a mechanical system for deaeration, pre-compression and extrusion; a cylinder head for guiding a piston; mechanical assembly for transmitting movement of the piston; back box and a casing.