The oxygen nonstoichiometry of SrFeO3- was determined by solid-state Coulometric titration at 750 degrees C-1040 degrees C and 10(-18)pO(2)(atm)0.5. At T850 degrees C, a hysteresis in the oxygen nonstoichiometry () isotherms indicates the presence of a two-phase region corresponding to a mixture of a perovskite and an oxygen vacancy ordered phase. The variation of with temperature at fixed pO(2) values and the variation of log(pO(2)) with reciprocal temperature at fixed are reported. The pO(2) at the p-n transition in the electrical conductivity increases as the temperature increases, and the transition in the conductivity isotherm occurs at a composition of SrFeO2.505(1) at 950 degrees C and SrFeO2.508(1) at 900 degrees C. Since strontium exclusively occupies the A-site, a simple point defect model that assumes noninteracting defects can describe the isotherms but only at 1040 degrees C; nonideal behavior is observed at lower temperatures. The partial molar thermodynamic quantities for oxygen in SrFeO3- were determined.