Abstract Because of the high economic, environmental, and safety costs associated with pure oxygen as a process feedstock, oxygen-conducting ceramic membranes have been explored as an alternative oxygen source for hydrocarbon conversion reactors. These nonporous ceramic materials allow the transport of oxygen ions through the lattice of the solid material, resulting in 100% selectivity for oxygen regardless of the source gas. The materials of interest for membrane reactor applications are mixed ionic–electronic conducting (MIEC) ceramic materials that conduct electrons as well as oxygen ions. As a result, they require only high temperatures (typically 700 °C or higher) and an imposed oxygen potential gradient to transmit oxygen. This chapter describes general attributes of oxygen-MIEC membranes, followed by an overview of published work with oxygen-MIEC ceramic membranes. Specific topics include membrane compositions, membrane modification strategies, and applications of oxygen-MIEC membranes for synthesis gas production.