The oxygen reduction mechanism and time evolution behavior were studied for porous La0.5-xPrxBa0.5CoO3-δ (LPBC) electrodes deposited onto Ce0.9Gd0.1O1.95 electrolytes. Electrochemical impedance spectroscopy (EIS) measurements, performed between 400 and 800°C as a function of pO2, show that the electrochemical response is limited by O2-gas diffusion and surface reaction (Osurface exchange + O-ion diffusion). The time evolution of the cathode resistance at 700°C in air shows an increase from 0.03 to 0.05 Ωcm2 after 280 h of testing. Three-dimensional (3D) tomographic using focused ion beam-scanning electron microscopy (FIB-SEM) showed little changes of microstructure after the ageing. The increase in resistance was explained by an increase in the amount of a water-soluble Ba-rich surface phase by a factor of 2, after ageing of the LPBC electrode, by an etching procedure coupled with inductively coupled plasma-optical emission spectrometry (ICP-OES).