The influence of Ca and Co doping in the Ruddlesden - Popper series of oxide Sr(2.7-x)Ca(x)Ln(0.3)Fe(2-y)Co(y)O(7-delta) with x = 0 and 0.3, y = 0 and 0.6, and Ln = La and Nd on the electrical and thermal properties, catalytic activity for the oxygen reduction reaction (ORR) in solid oxide fuel cells (SOFC), and phase stability has been investigated. The Co-substituted samples display an increase in oxygen vacancies, electrical conductivity, thermal expansion coefficient, and electrocatalytic activity for both Ln = La and Nd. Although Ca doping in the Sr site slightly reduces the oxygen loss and thermal expansion coefficient, the symmetric cell performance is improved for both lanthanides at high temperatures. Among the cathode materials in this study, the highest catalytic activity for the ORR is achieved with Sr2.7Ca0.3Nd0.3Fe1.4Co0.6O7.delta + Gd0.2Ce0.8O1.9 (GDC) composite cathode with an area specific resistance of 0.034 Omega cm(2) at 800 degrees C. Long-term thermal stability test shows that no impurity forms when Sr(2.7-x)Ca(x)Ln(0.3)Fe(2-y)Co(y)O(7-delta) oxides were heated at 800 degrees C for 150 h in air. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.