Non-porous oxygen-conducting ceramic membranes with mixed ionic and electronic conductivity (MIECs) have received significant interest for use as oxygen supplies in membrane reactor systems as a way to eliminate costly air separation steps and to achieve staged addition of oxygen. Previous studies that focused on the use of ceramic MIEC membranes for the partial oxidation of methane have shown that the presence of a catalyst near the permeate surface of a membrane can increase oxygen flux up to ten times by reducing the permeate side oxygen partial pressure. However, it has proven to be difficult to identify membrane materials and fabrication approaches that guarantee both high oxygen flux and mechanical stability. The investigations that have grown out of this difficulty have produced novel ceramic materials that could lead to membranes with both high flux and stability, but they have also exposed some intriguing uncertainties, such as the relative importance of surface reaction versus ion diffusion for oxygen transport across a given membrane and the transient behavior of MIEC materials in changing environments. This work presents the results from studies undertaken to clarify the role of membrane fabrication conditions and true surface area (as determined by AFM and profilometry) on oxygen flux through the non-perovskite mixed conducting SrFeCo0.5O3.25-x (SFC) membrane material. Additionally, semiconductor processing techniques were used to deposit catalyst particles and thin catalyst films directly on the cathodic oxygen dissociation surface of SFC membranes and the resulting performance enhancements are discussed. These results indicate that oxygen transport across an SFC membrane, although previously considered to be bulk diffusion limited, is actually controlled by both surface and bulk material properties, which encourages careful re-examination of other materials that may have been prematurely categorized. Performance data for newer ceramics, including the perovskites Ba0.5Sr0.5Co0.8Fe0.2O3-x and BaCo0.4Fe0.4Zr0.2O3-x, are reported from identical test conditions as the SFC tests to demonstrate rigorously the relative performance of more advanced MIEC materials. Finally, the transient behavior of these membrane materials under relevant operating conditions were explored using TGA/DSC and mass spectrometry to illuminate important material attributes that could facilitate the ongoing search for higher flux, more stable me

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