A time-dependent three-dimensional (3D) impedance model of mixed ionic electronic conducting solid oxide fuel cell (SOFC) cathodes that considers the complex coupling of gas diffusion, surface exchange, ionic bulk-diffusion and electrolyte conductivity is presented. By using the finite element method, this model enables the time-dependent and space-resolved simulation of the physicochemical processes in a porous cathode microstructure. The developed model is used for a detailed analysis of the formation of a 'Gerischer-type' impedance. It is detected that the low-frequency part is dominated by the surface exchange reaction, whereas the typical 45 degrees ramp of the Gerischer impedance is related to the ionic diffusion in the bulk. The capability of the time-dependent 3D impedance model is evaluated versus a well-established homogenized analytical model. For homogeneous 3D microstructures both models calculate impedance curves which are in excellent agreement. Further impedance simulations with microstructures containing features of high-performance SOFC cathodes clearly show that model separates and quantifies the contribution of the gas diffusion in a porous cathode layer. At an oxygen partial pressure of 0.21 atm the gas diffusion accounts for only 2% of the total polarization resistance, whereas a depletion of oxygen to 0.01 atm significantly increases this value to 38%. (C) 2013 The Electrochemical Society. All rights reserved.