The oxygen reduction reaction performance of La0.6Sr0.4Co0.2Fe0.8O3-delta-Sm0.2Ce0.8O2-delta composites is associated with the surface micro-structure of the dual phases. This work prepares the composites with various SDC grain sizes to reveal the influence of ionic phase micro-structure on the surface reaction using the electrical conductivity relaxation (ECR) method. Based on the quantitatively counted micro-structure characteristics, the enhanced surface reaction rate is found to increase with the raised SDC grain size. The results suggest that the incorporation reaction is not restricted at LSCF-SDC-gas three-phase boundary (3PB). Another reaction route is available with the spillover of adsorbed oxygen species on the SDC particle surface to combine with the abundant oxygen vacancy. And the diffusion distance of overflowed oxygen species is proven to be over 0.42 mu m according to the experimental results. (C) 2015 The Electrochemical Society. All rights reserved.