Mixed-conducting Sr-Fe-Co oxides have potential applications in dense ceramic membranes for high-purity oxygen separation and/or methane conversion to produce syngas (CO + H-2), because of their combined high electronic/ionic conductivity and significant oxygen permeability. SrFeCoo(0.5)O(y) has been synthesized by the solid-state reaction method. Conductivities were measured at elevated temperatures in various gas environments and increased as temperature and oxygen partial pressure (p(O2)) increased in the surrounding environment. Neutron powder diffraction experiments revealed that in a high p(O2) environment the SrFeCo0.5Oy material consists of three different phases. The relative concentration of each component phase is dependent on temperature and p(O2) in the surrounding environment. In air, Sr-2(Fe,CO)(3)O-y (236-phase) is the majority phase and consists of > 75 wt% of the total, whereas the perovskite and rocksalt phases account for similar to20 and < 5 wt%, respectively. However, in a reducing environment, the 236-phase decomposes and converts to perovskite and rocksalt phase at high temperature. In an environment of p(O2) < 10(-12.2) atm, the 236-phase is completely converted into perovskite (brownmillerite) and rocksalt phases.