Mixed-conducting ceramic oxides have potential uses in hightemperature electrochemical applications such as solid-oxide fuel cells, batteries, sensors, and oxygen-permeable membranes. The Sr 4 (Fe 1؊x Co x ) 6 O 13؎ system combines high electronic/ionic conductivity with appreciable oxygen permeability at elevated temperatures. Dense ceramic membranes made of this material can be used to separate high-purity oxygen from air without the need for external electrical circuitry or to partially oxidize methane to produce synthesis gas (syngas, CO ؉ H 2 ). Samples of Sr 4 (Fe 1؊x Co x ) 6 O 13؎ (where x ؍ 0, 0.1, 0.2, 0.333, and 0.467) were prepared by a solid-state reaction method in atmospheres with various oxygen partial pressure (p O 2 ) and were characterized by powder X-ray diffraction, scanning electron microscopy, and electrical conductivity testing. The proportion of phase components in the samples is dependent on both cobalt content and p O2 . The total conductivity increases with both temperature and cobalt substitution in the material. Current-voltage characteristics determined in a gas-tight cell indicate that a bulk effect, rather than a surface exchange effect, is the main limiting factor for oxygen permeation through membranes made of Sr 4 Fe 4 Co 2 O 13؎ (x ؍ 0.333 sample). Oxygen permeability measurements at various temperatures showed that, as expected, permeability increases with increasing temperature. At 900°C, an oxygen permeation flux of 2.5 scc ) cm ؊2 ) min ؊1 was obtained from an Sr 4 Fe 4 Co 2 O 13؎ disk membrane that was 2.9 mm thick.