La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) symmetric-electrode cells with Gd0.1Ce0.9O1.95 (GDC) electrolytes were aged in ambient air at temperatures ranging from 550 to 950 degrees C for times up to 1400 h, without current/polarization. Electrochemical impedance spectroscopy measurements, taken periodically during the tests at a testing temperature of 700 degrees C for cells aged at 700, 750 and 800 degrees C, showed a continuous increase in polarization resistance. Focused ion beam-scanning electron microscopy (FIB-SEM) 3D tomography showed that the LSCF electrode did not coarsen measurably at ageing temperatures <= 800 degrees C, ruling out LSCF microstructural changes as the mechanism behind the resistance increase. On the other hand, Sr surface segregation, determined by chemical etching with inductively coupled plasma-optical emission spectrometry (ICP-OES) detection, was found to increase with increasing ageing time and temperature > 650 degrees C. The effect of Sr surface segregation on the oxygen surface exchange and diffusion processes is quantified by comparing the measured electrochemical and microstructural data. (C) 2018 The Electrochemical Society.