Sm0.5Sr0.5CoO3 (SSC) porous electrodes supported on La0.9Sr0.1Ga0.8Mg0.2O3 were studied using X-ray diffraction, scanning electron microscopy, impedance spectroscopy, and dc polarization. The addition of La0.8Sr0.2Ga0.8Mg0.15Co0.05O3 (LSGMC5) or La0.8Sr0.2Ga0.8Mg0.09Co0.11O3 (LSGMC11) into SSC improved its performance significantly. Two arcs could be identified in the impedance spectra of oxygen reduction over SSC electrodes. The conductivities of the high-frequency arcs had no dependency on oxygen partial pressures (p(O2)), which could be related to the transport of oxygen ions. While those of the low-frequency arcs had a p(O2) dependency of about 0.5 at 1073 K, corresponding to a rate-determining step of the diffusion of oxygen atoms at the surface of the electrodes. DC polarization curves of SSC electrodes agreed with the well-known Butler-Volmer equation. Both the cathodic and anodic charge-transfer coefficients were about 1, and the dependencies of the exchange current densities on p(O2) were about 0.25, suggesting a rate-determining step of charge transfer. It seemed that the mechanism of oxygen reduction on SSC electrodes depended strongly on reaction conditions. (C) 2004 The Electrochemical Society.