The degradation of the permeation flux of Ba0.5Sr0.5Co0.8Fe0.2O3-delta membranes has typically been attributed to the phase transformation of the material at intermediate temperatures. In this study, the effect of the interfacial oxygen exchange steps was considered to give an overall view of the degradation mechanism. The changes in the interfacial exchange resistances, bulk resistance, and morphologies of the membranes were investigated via physical characterizations and a permeation model. The interfacial oxygen exchange resistances increased more quickly with time than bulk resistance. Meanwhile, BaSO4 particles were detected on both surfaces of the membranes, and their contents reached maximum at 650 degrees C. However, after the membrane surfaces were coated by Sm0.5Sr0.5CoO3-delta porous layers, the interfacial oxygen exchange resistances kept constant and the degradation rates were slowed down. The degradation was predominated by the increase of interfacial oxygen exchange resistances induced by the enrichment of BaSO4 particles on membrane surfaces. (c) 2015 American Institute of Chemical Engineers