The electrochemical properties of the interfaces between porous composites of La0.6Sr0.4Co0.2Fe0.8O3-delta/Ce0.9Gd0.1O2-delta cathodes and Ce0.9Gd0.1O2-delta electrolytes have been investigated at intermediate temperatures (500-700 degrees C) using AC impedance spectroscopy. Results indicate that the electrochemical properties of these composites are quite sensitive to the composition and the microstructure of the electrode. The optimum Ce0 9Gd0.1O2-delta addition (36% by volume) to La0.6Sr0.4Co0.2Fe0.8O3-delta resulted in four times lower area specific resistivity, which classify this composite as a promising material for solid oxide fuel cells based on Ce-0 9Gd0.1O2-delta electrolytes. The observed high performance of the composite electrodes at this composition is consistent with the effective medium percolation theory which predicts the ambipolar transport behaviour of composite mixed ionic-electronic conductors as a function of the volume fraction of each of the randomly-distributed constituent phases. Quantitatively, a slight discrepancy between measurements and theory was observed. This is believed to be due to the fact that the overall performance of a porous electrode is not only determined by the mixed conducting transport properties in the solid phase of the electrode, but also by the inherent catalytic property of the triple phase boundary, and by the gas transport to, or away, from the triple phase boundary.