The large-scale commercialization of SOFCs is currently constrained by a combination of cost and durability issues. In order to address these concerns, much effort have been focused on reducing the cell operating temperature below 700 °C [1]. One of the major issues to achieve this goal is the development of cathode materials with low polarization resistance and good stability over time. Cobaltites with perovskite structure, and mainly the Strontium-Barium cobaltites (BSCF) [2], are one of the most promising cathode materials for IT-SOFCs. Despite the good performance of Sr-containing perovskites, one major issues is the long-term instability due to Sr-surface segregation [3]. However, as far as we know, the possibility of degradation due to Ba-segregation has not yet been analyzed in Ba-cobaltites. In these perovskites, the Ba plays a key role since its large cation radii distorts the cubic crystal structure promoting the oxygen vacancy formation and migration [4] reducing the Rp,c due to the improvement of the O-surface exchange and the O-ion diffusion. However, the same structural distortion, also induces a slow segregation of a hexagonal perovskite phase [5] which deteriorates the O2 reduction kinetics with time. Recently, we showed that La0.5Ba0.5CoO3−δ is a promising cathode materials for IT-SOFCs because of their low cathode polarization resistance [6]. In this oxide, we did not observe the hexagonal phase formation, suggesting that La+3ion because of its charge and ionic radii play an important role stabilizing the cubic phase. With this idea, in this work, the La content of the perovskite oxide La0.5Ba0.5CoO3−δ was partially substitute with Praseodymium as La0.5-xPrxBa0.5CoO3-δ, with 0 < x < 0.5 with the aim of improve even more the cathode performance. Samples exhibit cubic symmetry for x ≤ 0.35, whereas above this value the cation ordering produce a layering structure with tetragonal symmetry. The Electrochemical Impedance Spectroscopy (EIS) as a function of temperature (T), oxygen partial pressure (pO2) and time in combination with 3D FIB-SEM tomography and the Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) studies were used to analyze the mechanism of O2 reduction and the origin of degradation with time. The study of the oxygen reduction reactions (ORR) mechanisms by EIS were carried out in air as a function of temperature between 400 and 800 ºC. In addition, EIS spectra were collected at 700 ºC varying the oxygen partial pressure (pO2) between 1 and 5 × 10−4 atm. All samples showed similar behavior. As an example, at 700 ºC in air the ORR for La0.30Pr0.2Ba0.5CoO3-δ, with a cathode polarization resistance Rp,c ~ 0.04 Ωcm2, is mainly dominated by O2-gas diffusion. This contribution remains almost constant with time, whereas the high frequency contribution associated to the surface reaction (O-surface exchange + O-ion diffusion) increases with time. Tested samples at 700 ºC after 250 hours in air and fresh sample were analyzed by FIB-SEM tomography and ICP-OES to investigate possible changes in the microstructure and cation segregation in the surface. Again, all samples showed similar behavior with time, i.e. the overall rate of change, defined as Rp,c,0 -1×(dRp,c/dt), is 0.003 h-1 for La0.35Pr0.15Ba0.5CoO3-δ. The comparison between this sample before and after aging test showed that the main difference is not about microstructure change, but in the Ba-surface concentration, which is duplicated. References: [1] Gao, Z., Mogni, L.V., Miller, E.C., Railsback, J.G.,Barnett, S.A. A perspective on low-temperature solid oxide fuel cells (Review). Energy and Environmental Science. Volume 9, Issue 5, May 2016, Pages 1602-1644. [2] S.M.H. Zongping Shao, Nature 431 (2004) 170–173. [3] S.P. Simner, M.D. Anderson, M.H. Engelhard, J.W. Stevenson, Electrochemical and Solid-State Letters 9 (10) (2006) A478. [4] R. Merkle, Y.a. Mastrikov, E.a. Kotomin, M.M. Kuklja, J. Maier, J. Electrochem. Soc. 159 (2012) B219–B219. [5] D.N. Mueller, R.A. De Souza, T.E. Weirich, D. Roehrens, J. Mayer, M. Martin, Phys. Chem. Chem. Phys.: PCCP 12 (2010) 10320–10328 [6] D. Garces, C.F. Setevich, A. Caneiro, G. Cuello, L. Mogni, J. Appl. Crystallogr. 47 (2014) 325–334.