Mixed-oxide conductors find wide application in high-temperature, solid-state electrochemical devices such as solid-oxide fuel cells, batteries, and sensors. By using a gastight electrochemical cell with flowing air as the reference environment, the authors were able to achieve an oxygen partial pressure (P{sub O{sub 2}}) and temperature. It was found that in the high-P{sub O{sub 2}} range, interstitial oxygen ions and electron holes are the dominant charge carriers, while in the low-P{sub O{sub 2}} range, oxygen vacancies and electrons are dominant. At 800 C in air, total conductivity and ionic conductivity of SrFeCo{sub 0.5}O{sub x} are 17 and 7 S cm{sup {minus}1}, respectively, and the ionic transference number is 0.4. A semiconductor-metal-semiconductor transition is found in this system in a reduced-oxygen environment. Defect dynamics in this system can be understood by means of the trivalence-to-trivalence transition of Fe ions when P{sub O{sub 2}} is reduced. A defect model has been proposed. By using the conductivity results, the authors were able to estimate oxygen permeation through a ceramic membrane made of Sr FeCo{sub 0.5}O{sub x}. The oxygen permeability they calculated is consistent with that measured at the conversion reactor. To confirm the ionic transference number measured by electron-blocking method, electromotive force measurementmore » was carried out and obtained consistent results.« less