Dense ceramic tubes of the multi-phase mixed ionic/electronic conductor SrFeCo0.5Oy have been synthesized by solid-state reaction, and the stability of its component phases studied using in-situ neutron diffraction over the oxygen partial pressure (pO(2)) range 10(-0.68) to 10(-15.0) atm at 900 degrees C, This being the likely conditions to which membranes are exposed in operational methane conversion reactors. This material is stable between air and log pO(2) = -2.3, with Sr-2(Fe,Co)(3)O-y (236) being the majority phase. When the oxygen partial pressure is lowered, 236 starts decomposing into perovskite and rocksalt phases. The small fraction of remaining 236 rapidly undergoes decomposition at log pO(2) = -12.2, leaving only SrFeO2.5 and CoO. Rietveld analysis indicates that the oxygen content of the perovskite phase does not drop below O = 2.5 when log pO(2) = -15.0, while there is an indication that CoO is reduced to cobalt metal. This decomposition at low partial pressures may be reversed when the gas is switched back to air, i.e., SrFeO2.5 + CoO re-form Sr-2(Fe,Co)(3)O-y, to an extent that is seemingly dependent on sample and thermal history.