Abstract The impedance of technological relevant LSCF:CGO composite IT-SOFC cathodes was studied over a very wide performance range. This was experimentally achieved by impedance measurements on symmetrical cells with three different microstructures in the temperature range 550–850 °C. In order to account for the impedance spectra of the poor performing cathodes the Finite-Length-Gerischer (FLG) impedance was derived and applied to the impedance data. The FLG impedance describes for a given microstructure the situation where the cathode is made too thin from a cathode development point of view. The moderate performing cathodes showed a slightly suppressed Gerischer impedance, while the impedance spectra of the well performing cathodes showed the presence of an arc due to oxygen gas diffusion. The overall impedance of the well performing cathodes could be described with a slightly suppressed Gerischer impedance element in series with a Finite-Length-Warburg (FLW) impedance element. Finally, the origin to a suppression or distortion of the FLG and the Gerischer impedance was discussed and explored in relation to e.g. numerical simulations on the effect of a slightly distributed diffusion length in the FLG, due to different torturous diffusion pathways within the cathodes.