Experimental data on half-cells consisting of YSZ electrolyte pellets and slurry-coated cathodes, and a simplified theoretical model were used to give an insight into the kinetics of oxygen reduction in solid electrolyte composite cathodes. Electrochemical impedance spectroscopy and potentiodynamic polarizations were used to evaluate the main electrochemical parameters of the cathodic process in a temperature range between and . The experimental results show that the oxygen reaction is not under activation control at low temperatures, and other phenomena, such as the transfer of oxygen ions to and through the solid electrolyte in the composite cathode, occur and retard the overall rate of oxygen reduction. This working hypothesis was assessed using a simplified theoretical model of the cathode that accounts for charge transfer, mass transfer, and conduction. The model simulations compared satisfactorily with the experimental data, and they show that, at low temperatures, the reaction zone in the cathode is confined to the electrolyte interface. When the temperature is increased, the retarding effects of mass transfer and conduction in the electrolyte become negligible, and the reaction zone progressively extends through the electrode.