Perovskite-related intergrowth oxides Sr(2.7)Ln(0.3)Fe(1.4)Co(0.6)O(7-delta) (Ln=La, Nd, Sm, and Gd) have been investigated as cathode materials for solid, oxide fuel cells (SOFC). With decreasing size of the Ln(3+) ions, the unit cell volume, oxygen content, thermal expansion coefficient (TEC), and total electrical conductivity decrease from Ln = La to Gd. The decreasing unit cell volume and oxygen content is attributed to the decreasing size of Ln(3+) ions from Ln=La to Gd and a consequent preference for lower coordination numbers. While the decrease in the ionicity of the Ln-O bonds from Ln = La to Gd causes a decrease in the TEC, the increasing amount of oxygen vacancies leads to a decrease in electrical conductivity arising from a thermally activated semiconducting behavior. The cathode polarization conductance (R-p(-1)) measured using the ac-impedance spectroscopy and the catalytic activity for the oxygen reduction reaction in SOFC decrease from Ln = La to Gd partly due to the decrease in electrical conductivity. (C) 2009 Elsevier B.V. All rights reserved.