Composites of Sm0.5Sr0.5CoO3-delta (SSC), a good electronic conductor, and La0.8Sr0.2Ga0.8Mg0.2O3-delta (LSGM), a good ionic conductor, have been evaluated as cathode systems for La-gallate electrolyte based SOFC systems via microstructural analysis and electrochemical testing. SSC and LSGM composite cathode half-cells were fabricated with varying phase fraction ratios. X-ray diffraction and transmission electron microscopy studies show that these electrode and electrolyte materials will inter-diffuse with long-term exposure in service environments. Within an electrode, inter-diffusion occurs between SSC and LSGM particles of varying initial particle sizes, resulting in heterogeneity in the local cation ratios. Of the SSC/LSGM weight ratios analyzed (pure SSC, 80:20, 70:30, and 50:50), the 80:20 ratio was identified to possess the lowest resistance, according to AC impedance results. The microstructural basis of this finding, and implications for optimal materials design, are discussed. (C) 2013 Elsevier B.V. All rights reserved.