The mixed-conducting SrFeCo0.5Ox is a potential material for use in dense ceramic membranes for gas separation at elevated temperatures. Its outstanding oxygen transport properties reveal the feasibility of using oxide ceramic membrane in large-scale oxygen separation. The stability of this material under low oxygen partial pressure is an important issue. To investigate phase and structural stability, SrFeCo0.5Ox samples were annealed in various reducing gaseous environments. The annealed samples were characterized by room-temperature X-ray powder diffraction and scanning electron microscopy analyses. In situ conductivity of the samples was measured in various atmospheres. Reversible phase transition related to the change of oxygen partial pressure in the surrounding atmosphere was observed at elevated temperatures. Our results show that mechanical failure of dense ceramic membranes made of this material may occur when it is cooled in a moisturized reducing atmosphere, such as hydrogen, and then reheated in an oxidizing atmosphere, such as air.