The kinetics of oxygen reduction over various Sm0.5Sr0.5CoO3(SSC)-La0.8Sr0.2Ga0.8Mg0.15Co0.05O3(LSGMC5)/LSGMC5 (interlayer)/LSGMC5 (electrolyte) assemblies were studied, which were essential to find the role of an interlayer in improving the performance of an electrode/electrolyte interface. Two major arcs were identified in the impedance spectra at near equilibrium conditions. The reciprocal of the electrode resistance corresponding to the high frequency arc showed a P-O2 dependency about 0.5 at 1073 K and decreased to one-fourth at 873 K, suggesting that the rate-determining step (rds) changed from the dissociative adsorption of oxygen or diffusion of adsorbed oxygen atoms to charge transfer. The reciprocal of the electrode resistance corresponding to the low frequency arc showed a P-O2 dependency about 1, suggesting an rds involving the gas diffusion of oxygen. DC polarization curves of various assemblies agreed well with the Butler-Volmer equation. Both the cathodic and anodic charge transfer coefficients were about 1, and the P-O2 dependencies of the exchange current densities were about 0.25, especially at low temperatures. The characteristics under polarization corresponded to a charge transfer process. The introduction of an LSGMC5 interlayer between the SSC-LSGMC5 electrode and LSGMC5 electrolyte did not change the reaction mechanism, and the role of the interlayer was to increase the number of active sites for oxygen reduction. (c) 2006 Elsevier Ltd. All rights reserved.