Solid-state coulometric titration has been used to determine the oxygen non-stoichiometry of La0.5Sr0.5FeO3-x at 800less than or equal toTless than or equal to1000 degreesC and 10(-16)less than or equal torhoO(2)less than or equal to0.5 atm. Above 900 degreesC, reproducible data were obtained on both increasing and decreasing rhoO(2). At temperatures Tless than or equal to850 degreesC, a discontinuity appears in the variation of the oxygen non-stoichiometry with rhoO(2), indicative of two-phase behavior. The electrical conductivity of La0.5Sr0.5FeO3-x, was determined at 800 and 900 degreesC and 10(-14)less than or equal torhoO(2)less than or equal to0.3 atm by a FOUR-probe ac technique. Slow equilibration kinetics were observed in the intermediate rhoO(2) region (similar to10(-5) atm) due to either short-range order of oxygen vacancies or phase separation between a perovskite phase and a vacancy ordered phase. The isotherms are reproducible, irrespective of the direction of the rhoO(2) change, provided long enough equilibration times are used. The criteria for equilibrium used in the experiments are discussed. (C) 2004 Elsevier B.V All rights reserved.