Mixed-conducting ceramics have the ability to conduct oxygen with perfect selectivity at elevated temperatures, making them extremely attractive as membrane materials for oxygen separation and membrane reactor applications. While the conductivity of these materials can be quite high at elevated temperatures (typically 800-1000 C), much higher oxygen fluxes, or, alternatively, equivalent fluxes at lower temperatures, could be provided by supported thin or thick film membrane layers. Based on that motivation, the objective of this project was to explore the use of ultrafine aerosol-derived powder of a mixed-conducting ceramic material for fabrication of supported thick-film dense membranes. The project focused on the mixed-conducting ceramic composition SrCo[sub 0.5]FeO[sub x] (SCFO) because of the desirable permeability and stability of that material, as reported in the literature. Appropriate conditions to produce the submicron SrCo[sub 0.5]FeO[sub x] powder using aerosol pyrolysis were determined. Porous supports of the same composition were produced by partial sintering of a commercially obtained powder that possessed significantly larger particle size than the aerosol-derived powder. The effects of sintering conditions (temperature, atmosphere) on the porosity and microstructure of the porous discs were studied, and a standard support fabrication procedure was adopted. Subsequently, a variety of paste and slurry formulations were explored utilizing the aerosol-derived SCFO powder. These formulations were applied to the porous SCFO support by a doctor blade or spin coating procedure. Sintering of the supported membrane layer was then conducted, and additional layers were deposited and sintered in some cases. The primary characterization methods were X-ray diffraction and scanning electron microscopy, and room-temperature nitrogen permeation was used to assess defect status of the membranes. We found that non-aqueous paste/slurry formulations incorporating dispersant, plasticizer and binder provided superior cracking resistance compared to simple water, alcohol, or polyethylene glycol (PEG) based formulations. With a formulation employing castor oil as dispersant, isopropyl alcohol/mineral spirits as solvent, polyvinyl butyral as binder, and dibutyl phthalate/PEG as plasticizer, sintered SCFO membrane layers approximately 5[micro]m thick with no apparent cracks were prepared using spin coating with several coats and sintering cycles. A similar but more viscous formulation applied by doctor blade gave a[approx] 10[micro]m thick membrane layer in one coat, but with some apparent cracking. We demonstrated that the membrane layer could be densified while retaining porosity in the chemically identical support. This was accomplished by pre-sintering the support in air (1050 C), which coarsened the grain size and provided a relatively stable plate-shaped granular microstructure, followed by membrane layer fabrication with the highly-sinterable aerosol powder. Final densification was conducted by sintering in nitrogen ([approx]1100 C), which provided accelerated sintering rates and led to the desired layered perovskite phase content. In spite of these successes, low-temperature pressure-driven permeation testing with N2 showed that even the best membranes were not sufficiently defect free for high-temperature oxygen permeation testing. The source of these defects were not readily apparent from scanning electron microscopy, though incomplete or nonuniform membrane layer coverage from edge to edge of the support was probably one important factor.