Symmetric cells with porous La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) electrodes on Gd0.1Ce0.9O1.95 (GDC) electrolytes were aged at 800◦C for 800 hours in ambient air. Electrochemical impedance spectroscopy (EIS) measurements performed periodically at 700◦C showed a continuous increase of the polarization resistance from 0.15 to 0.34 􏰀 · cm2 . Three-dimensional (3D) tomographic analysis using focused ion beam–scanning electron microscopy (FIB-SEM) showed negligible changes due to the ageing, suggesting that the observed resistance increase was not caused by electrode morphological evolution. However, an increased amount, by a factor of 3, of a water-soluble Sr rich surface phase on the aged LSCF electrode was detected by an etching procedure coupled with inductively coupled plasma-optical emission spectrometry (ICP-OES). The electrochemical analysis in combination with the microstructural parameters determined by FIB-SEM was used to examine the effect of Sr segregation on the rate of oxygen surface exchange, based on the Adler-Lane-Steele (ALS) model.