We present a combined modeling and experimental study of electrochemical oxygen reduction at mixed-conducting composite LSCF/CGO solid oxide fuel cell (SOFC) cathodes. The developed kinetic model incorporates elementary heterogeneous chemistry and electrochemical charge-transfer processes at two different electrochemical double layers, transport in the porous composite electrode (ionic and electronic conduction, multi-component porous diffusion and convection) as well as gas supply. A full set of thermodynamic and kinetic parameters is developed. Experimentally, La0.6Sr0.4Co0.8Fe0.2O3-δ/Ce0.9Gd0.1O2-α composite electrodes embedded into a symmetrical cell with CGO electrolyte were characterized via electrochemical impedance spectroscopy. The model shows good agreement with experimental impedance data over the complete range of investigated conditions (temperature range 775 K–1075 K, frequency range 10 mHz–100 kHz). This allows a mechanistic interpretation of the origin of the three observed impedance features: (i) low frequency: transport in the gas supply (gas conversion), (ii) intermediate frequency: charge transfer and surface double layer at the LSCF/air interface, (iii) high frequency: charge transfer and electrical double layer at the LSCF/CGO interface.