The cathode reaction mechanism of porous Sm0.5Sr0.5CoO3-delta, a mixed ionic and electronic conductor (MIEC), is studied through a comparison with the composite cathode Sm0.5Sr0.5CoO3-delta/Sm0.2Ce0.8O1.9. First, the cathodic behaviour of porous Sm0.5Sr0.5CoO3-delta and Sm0.5Sr0.5CoO3-delta/Sm0.2Ce0.8O1.9 are observed for micro-structure and impedance spectra according to Sm0.2Ce0.8O1.9 addition. thermal cycling and long-term properties. The cathode reaction mechanism is discussed in terms of frequency response, activation energy. reaction order and electrode resistance for different oxygen partial pressures p(O-2) at various temperatures. Three elementary steps are considered to be involved in the cathodic reaction: (i) oxygen ion transfer at the cathode-electrolyte interface; (ii) oxygen ion conduction in the bulk cathode; (iii) gas phase diffusion of oxygen. A reaction model based on the empirical equivalent circuit is introduced and analyzed using the impedance spectra. The electrode resistance at high frequency (R-c.HF) in the impedance spectra represents reaction steps (i), due to its fast reaction rate. The electrode resistance at high frequency is independent of p(O-2) at a constant temperature because the semicircle of R-c.HF in the complex plane of the impedance spectra is held constant for different values of p(O-2). Reaction steps (ii) and (iii) are the dominant processes for a MIEC cathode, according to the analysis results. The proposed cathode reaction model and results for a solid oxide fuel cell (SOFC) well describe a MIEC cathode with high ionic conductivity, and assist the understanding of the MIEC cathode reaction mechanism. (c) 2009 Elsevier B.V. All rights reserved.