Porous and dense Lao^Sro^CoCb-« layers were prepared as working electrodes on 10 mole % CaO doped CeCh electrolytes. Electrochemical measurements were made as a function of oxygen partial pressure at 500 to 900TS. The electrode interface conductivity and the steady-state polarization curve suggested that the reaction rates with porous electrodes are controlled by gas diffusion process at low P(O2) and those with dense electrodes by surface reaction.