Sr0.7R0.3CoO3-delta (R = Tb and Er) tetragonal perovskites have been prepared and evaluated as mixed ionic-electronic cathodes for SOFC. Neutron powder diffraction (NPD) measurements evidenced that both compounds are oxygen hypo-stoichiometric with long-range order of oxygen vacancies that leads to a tetragonal perovskite-type superstructure (s.g. I4/mmm) stable within the whole temperature range under study. The oxygen vacancies located mainly in the equatorial oxygen positions exhibit large displacement factors. The high oxygen mobility in Sr0.7Tb0.3CoO3-delta was confirmed by O-18 oxygen labeling followed by Secondary Ion Mass Spectrometry (SIMS) with values of oxygen self-diffusion of 1.29 x 10(-10) cm(2)/s at 525 degrees C. Polarization resistances with LSGM as electrolyte gave values as low as 0.011 Omega.cm(2) and maximum output powers of 570 mW/cm(2) at 850 degrees C were obtained in test cells set in electrolyte-supported configuration. Electrical conductivity, thermal and chemical expansion and stability measurements confirm the potential of these materials as cathodes for SOFC. (C) The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. All rights reserved.