The solid oxide fuel cell symmetrical cells with porous cathodes for intermediate temperature applications have been studied under electrochemical polarization and oxygen feeding conditions using high-temperature in situ XRD (HT-XRD) method. Changes in the lattice parameters of La0.6Sr0.4CoO3-delta and La0.6Sr0.4Co0.2Fe0.8O3-delta were observed depending on the temperature (T), electrode potential (E) and oxygen partial pressure (p(O2)) applied. At fixed T and p(O2), the cathode potential noticeably influences the crystallographic cell volume calculated, thus, i.e., the oxygen stoichiometry, explained by the partial reduction of electrochemically labile Co-ionic centers in La0.6Sr0.4Co3-delta (LSC), La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) and mechanically mixed cathodes La0.6Sr0.4CoO3-delta + La0.6Sr0.4Co0.2Fe0.8O3-delta (50:50 wt%) under study. Noticeably higher effect of cathode potential on the cell volume has been established at lower p(O2). The dependence of crystallographic parameters obtained is in a good agreement with electrochemical impedance spectroscopy data, indicating that the electrocatalytic activity of cathode decreases with the rise of Fe ions concentration in LSCF. (C) 2013 The Electrochemical Society. All rights reserved.