Crystal structure, thermal expansion coefficient, electrical conductivity and cathodic polarization of compositions in the System Sm0.5Sr0.5 Co1-xFexO3-delta with 0 <= x <= 0.9 were studied as function of Co/Fe ratio and temperature, in air. Two phases, including an Orthorhombic symmetry for 0 <= x <= 0.4 and a cubic symmetry for 0.5 <= x <= 0.9, were observed in samples of Sm0.5Sr0.5Co1-xFexO3-delta at room temperature. The adjustment of thermal expansion coefficient (TEC) to electrolyte, which is one of the main problems of SSC, could be achieved to lower TEC values with more Fe substitution. High electrical conductivity above 100 S/cm at 800 degrees C was obtained for all specimens, so they could be good conductors as cathodes of IT-SOFC. The polarization behavior of SSCF as a function of Fe content was evaluated by means of AC impedance using LSGM electrolyte. It was discovered that the Area Specific Resistance (ASR) of SSCF increased as the amount of substitution of Fe for Co increased. When the amount of Fe reached to 0.4, the highest ASR was obtained and then the resistance started decreasing above that. The electrode with a composition Of Sm0.5Sr0.5Co0.2Fe0.8O3-delta showed high catalytic activity for oxygen reduction operating at temperature ranging from 700 to 800 degrees C. (c) 2006 Elsevier B.V. All rights reserved.