Subsolidus phase relations at ambient atmospheric pressure and elevated temperatures in the Ba 1− y Sr y MnO 3− x system were investigated by quenching, gravimetric, and X-ray diffraction methods. The system is not binary above ∼1035°C because of reactions with atmospheric oxygen. The air isolar, P O 2 = 0.2 atm, was characterized at 1225, 1375, 1490, and 1610°C. Seven oxygen-deficient phases including a perovskite phase characterize the system. Their stability depends on the values of y and x in Ba 1− y Sr y MnO 3− x . The cell dimensions of these phases expand as x increases at fixed y. These seven modifications can be retained in stoichiometric form by oxidation at lower temperatures.