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-delta/Ce0.9Gd0.1O2-alpha, composite electrodes embedded into a symmetrical cell with COO 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. (C) 2014 The Electrochemical Society. All tights reserved.