The influence of the partial cubic-to-hexagonal phase transformation of Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) on the creep behaviour of this perovskite-type oxygen transport membrane was investigated. Isothermal compressive creep tests were carried out in air in the temperature range 750-950 degrees C. Arrhenius plots of the creep rate revealed a significant hysteresis effect. During heating, the creep rate changed slightly up to about 850 degrees C and increased substantially above this temperature. Upon cooling from 950 degrees C it decreased in a nearly linear manner to temperature. Microstructural investigations support the conclusion that the hysteresis is linked with the growth and transformation kinetics of the hexagonal polymorph at the boundaries of cubic grains of BSCF. (C) 2011 Elsevier B.V. All rights reserved.