The phase diagram of the nonstoichiometric oxide SrCo0.8Fe0.2O3-delta has been determined as a function of temperature (550 degrees C less than or equal to T less than or equal to 900 degrees C) and oxygen partial pressure (0.015 atm less than or equal to pO(2) less than or equal to 1 atm) by thermogravimetric analysis. Two phases are observed, perovskite and brownmillerite, separated by a two phase region. Above 770 degrees C only the phase with the perovskite structure is stable. Over the range of conditions studied, the perovskite phase has maximum and minimum oxygen contents of 3-delta = 2.65 (550 degrees C, pO(2) = 1 atm) and 3-delta = 2.43 (850 degrees C, pO(2) = 5 x 10(-4) atm), respectively.