Ln(0.6)Sr(0.4)CoO(3-delta) (Ln=La, Pr, Nd, Sm, and Gd) perovskite oxides have been synthesized and compared as cathode materials for intermediate temperature solid oxide fuel cells (SOFC). Both the electrical conductivity and the oxide ion vacancy concentration decrease from Ln=La to Gd, which leads to a decrease in the electrocatalytic activity for oxygen reduction in SOFC. However, the thermal expansion coefficient (TEC) decreases favorably from Ln=La to Gd due to a decreasing ionicity of the Ln-O bond and a suppression of the tendency to lose oxygen from the lattice with increasing temperature. Therefore, Ln(0.6)Sr(0.4)CoO(3-delta) cathodes with an intermediate lanthanide ion such as Nd3+ or Sm3+ offer a trade-off between electrocatalytic activity and TEC. Also, the differences in dependence of the properties of the Ln(1-x)Sr(x)CoO(3-delta) and Ln(1-x)Sr(x)MnO(3) systems on the Ln(3+) ions are discussed.