Several perovskite-type oxides (ABO(3)) containing transition metals on the B-site show mixed (electronic/ionic) conductivity, These mixed-conductivity oxides are promising materials for oxygen-permeating membranes that can operate without electrodes or external electrical circuitry. Oxides in the system La-Sr-Fe-Co-O permeate large amounts of oxygen, and extruded tubes of these materials have been evaluated in a reactor operating at ca, 850 degrees C for direct conversion of methane into syngas (CO+H-2) in the presence of a reforming catalyst. Methane conversion efficiencies of > 99% were observed, and some of the reactor tubes have been operated for over 1000 h. Membrane tubes were fabricated from calcined powders by a plastic extrusion technique. Ceramic powders in the La-Sr-Fe-Co-O system were made by solid-state reaction of the constituent carbonates, oxides, and/or nitrates. The chemical-phase behavior of the ceramic powders with varying stoichiometries were studied by high-temperature in-situ X-ray diffraction (XRD) as a function of oxygen partial pressure. The sintered extruded tubes were also characterized by XRD and scanning electron microscopy.