To optimize the performance of porous cathodes on Molten-Carbonate Fuel Cells it is important to know the relative contributions of all polarization processes. In this paper the technique of ac Impedance is applied to study the phenomena of the cathodes which are made of NiO or LiCoO2. Both the complex porous electrodes as well as plane dense electrodes with a known and simple geometry are studied. To interpret the impedance spectra of the dense electrodes equivalent circuits are used. The circuits are derived from one-dimensional diffusion equations giving rise to a charge transfer resistance(s) and a homogeneous reaction impedance(s). For NiO one has to account for the semi conductivity of the oxide; for LiCoO2 the theoretical equivalent circuits have to be extended with a parallel connection of a Randle’s circuit. The measurements on porous electrodes are described using our three-phase homogeneous model, which is based on an averaging theory of porous media.