The polarization resistance (Rp) of the n = 3 Ruddlesden-Popper phases LnSr(3)Fe(1.5)Co(1.5)O(10-delta) with Ln = La, Nd and Pr has been studied by Electrochemical Impedance Spectroscopy as a function of both temperature and oxygen partial pressure. The three electrodes show Rp values comparable to other Perovskite-related materials considered as candidates for cathode of solid oxide fuel cells. The lowest polarization resistance for oxygen reduction was found for PrSr3Fe1.5Co1.5O10-delta. The impedance spectra of the three electrodes reveal the presence of two contributions which has been preliminarily identified based on the dependence of the polarization resistance of these contributions on oxygen partial pressure (pO(2)) and temperature. For the intermediate frequency contribution, the rate limiting processes would be a mixture of oxygen adsorption plus charge transfer at the electrode surface with oxide ion diffusion whereas the low frequency arc would correspond to the oxygen molecule adsorption and dissociation and oxygen gas diffusion.