Even though solid oxide fuel/electrolysis cells (SOFC/SOEC) are already commercially available, the effect of electrochemical polarization on the electrochemical properties and overpotentials of individual electrodes is largely unexplored. This is partly due to difficulties in separating anode and cathode impedance features and overpotentials of operating fuel cells. For this, we present a novel three-electrode geometry to measure single-electrode impedance spectra and overpotentials in solid oxide cells. With this new design, we characterise polarised porous La0.6Sr0.4FeO3−δ (LSF) electrodes by simultaneous impedance spectroscopy and ambient pressure XPS measurements. With physically justified equivalent circuit models, we can show how the overpotential-dependent changes in the impedance and XPS spectra are related to oxygen vacancy and electronic point defect concentrations, which deterimine the electrochemical properties. The results are overall in very good agreement with the key findings of several previous studies on the bulk defect chemistry and surface chemistry of LSF. They show for example the exsolution of Fe0 particles during cathodic polarisation in H2 + H2O atmosphere that decrease the polarization resistance by roughly one order of magnitude.