Mixed conducting materials have potential for application of the membrane reactor under large chemical potential gradients. A perovskite-related SrFeGa0.25O3-delta was prepared by the conventional solid-state reaction and its phase under air, He and 8% H-2 was analyzed by in-situ high-temperature XRD. The perovskite structure in air transformed to orthorhombic brownmillerite at 600 degrees C in He atmosphere, and to cubic brownmillerite at around 1000 degrees C. In 8% H-2, the SrFeGa0.25O3-delta decomposed into other phases that were completely different from the perovskite and the brownmillerite above 900 degrees C. The phase decomposition in hydrogen caused a drastic change both in electrical conductivity and surface morphology of the membrane. The electrical and oxygen-transporting characteristics correlated with crystal structure of SrFeGa0.25O3-delta were discussed. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.