Abstract The cathode reaction mechanism of porous Sm0.5Sr0.5CoO3−δ, a mixed ionic and electronic conductor (MIEC), is studied through a comparison with the composite cathode Sm0.5Sr0.5CoO3−δ/Sm0.2Ce0.8O1.9. First, the cathodic behaviour of porous Sm0.5Sr0.5CoO3−δ and Sm0.5Sr0.5CoO3−δ/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(O2) 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 (Rc,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(O2) at a constant temperature because the semicircle of Rc,HF in the complex plane of the impedance spectra is held constant for different values of p(O2). 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.