The stability of Sm0.5Sr0.5CoO3 (SSC) under reduction conditions is investigated to determine whether it can be used as a cathode material in on-planar type, single-chamber, solid oxide fuel cells. The techniques of X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy are used to reveal the reduction mechanism of SSC. Impedance spectroscopy analysis also provides a better understanding of the influence of decomposed SSC phases oil cathode performance. Decomposition of SSC occurs oil the surface by the formation of clot-shaped SrO. Co(OH)(2) and CoO oil lot) of the reduced SSC layer at 250 degrees C in 4% H2O-96% H-2- The SSC perovskite structure is destroyed at 350 degrees C ill pure hydrogen. There is a catastrophic microstructural change in which SSC is completely decomposed to SrO and CoO that cover the surface of Sm2O3. (C) 2009 Elsevier B.V. All rights reserved.