By using a gastight electrochemical cell with flowing air as the reference environment, we were able to achieve an oxygen partial pressure (P-O2) as low as 10(-16) atm inside the cell. The conductivity of SrFeCo0.5Ox has been studied as a function of P-O2 and temperature. It was found that in the high-P-O2 range, interstitial oxygen ions (O-i ") and electron holes (Fe-Fe(.) = h(.)) are the dominant charge carriers, while in the low-P-C2 range, oxygen vacancies (V-o(..)) and electrons (e’) are dominant. At 800 degrees C in air, total conductivity and ionic conductivity of SrFeCo0.5Ox are 17 and 7 S cm(-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-divalence transition of Fe ions when P-O2 is reduced. A defect model has been proposed. By using the conductivity results, we were able to estimate oxygen permeation through a ceramic membrane made of SrFeCo0.5Ox. The oxygen permeability we calculated is consistent with that measured at the conversion reactor. To confirm the ionic transference number measured by electron-blocking method, electromotive force measurement was carried out and obtained consistent results.