(Ba0.5Sr0.5) 1-xSmxCo0.8Fe0.2O3-delta (BSSCF; 0.05 <= x <= 0.20) compounds were synthesized and characterized by powder X-ray diffraction (XRD), thermogravimetric (TG), electrical conductivity, and thermal expansion coefficient (TEC) measurements. An emphasis is made on the effect of Sm-doping on the material's properties. XRD measurments showed that the BSSCF sample had a cubic perovskite structure, while TG measurements proved the loss of lattice oxygen during heating up. The electrical conductivity increased with temperature up to about 350-450 degrees C, and then decreased due to the loss of lattice oxygen. Additionally, the lattice oxygen loss caused the differences in electrical conductivity on heating up and cooling down. Electrical conductivity relaxation measurements were performed on the x = 0.10, 0.20 samples at 450, 470, and 500 degrees C, and showed that the diffusion coefficient decreased with increasing content of Sm. The TEC values of BSSCF were 19.5-20.1 x 10(-6) K-1 from 30 to 800 degrees C. (c) 2006 Elsevier B.V. All rights reserved.