The crystal chemistry of Sr1-x,LaxCo0.8Fe0.2O3-delta (0 less than or equal to x less than or equal to 0.4) has been investigated in the temperature range 20 less than or equal to T less than or equal to 900 degreesC and oxygen partial pressure range 10(-5) less than or equal to pO(2) less than or equal to 1 atm, employing X-ray diffraction, electrical conductivity, and thermogravimetric measurements. The samples adopt the perovskite structure with pseudo-cubic or cubic symmetry at room temperature. With a decreasing oxidation state of (Fe,Co) from 3.5 to 3.27+, the oxygen content increases from 2.75 to 2.84 as the La content increases from 0 to 0.4. Electrical conductivity and thermogravimetric measurements indicate a complete transformation from the cubic perovskite to the ordered orthorhombic brownmillerite phase (P/B transition) for the x = 0 and 0.1 samples as the oxygen vacancy concentration increases. For a given temperature (T = 550 degreesC, the equilibrium pO(2) at which the P/B transition occurs shifts to lower values as the La content increases. While phase analyses after annealing under vacuum or low pO, conditions as well as high-temperature X-ray diffraction indicate the formation of the browntrullerite phases for x less than or equal to 0.3, the x = 0.4 sample is found to be stable without forming the brownmillerite phase under the temperature and pO, conditions investigated. (C) 2004 Elsevier B.V. All rights reserved.