The degradation of cathode performances by chromium poisoning was studied at two cathodes [(La0.8Sr0.2)(0.95)MnO3 (LSM) and La0.8Sr0.2FeO3] on the Ce0.8Gd0.2O2 electrolyte. Cathodic currents were decreased by the supply of Cr vapors under a constant overvoltage (-0.3 V vs reference electrode) at 1073 K up to 300 h. The polarization resistances increased with time to the power of 1/2 and 1/4 in (La0.8Sr0.2)(0.95)MnO3 and La0.8Sr0.2FeO3, respectively. The distributions and the amounts of Cr in the porous cathodes were measured by secondary-ion mass spectrometry: A maximum concentration of Cr was about 1000 ppm at the LSM/Ce0.8Gd0.2O2 contact interfaces, while the surface concentration was about 100 ppm. In La0.8Sr0.2FeO3, the concentration of Cr is almost constant at values of 1000-1700 ppm inside the cathode. Among the supplied Cr vapors, about 7-14% of Cr atoms were reacted and condensed in the porous (La0.8Sr0.2)(0.95)MnO3, while 13-21% of Cr atoms were reacted in La0.8Sr0.2FeO3. Chemical reactivity and electrode reaction mechanism affect the Cr poisoning and degradation of cathode performance.