In this work, anode supported solid oxide fuel cells based on Ni/YSZ anode, 8YSZ electrolyte and (La, Sr)(Co, Fe)O3-delta cathode were analyzed by Electrochemical Impedance Spectroscopy (EIS) with the aim to identify the main electrochemical mechanism contributing to the polarization resistance. The method was validated by performing EIS analyses on reference samples at different temperatures and overvoltages. In order to investigate the different electrochemical contributions to the polarization resistance, the study was carried out by testing the cells at different operating conditions (gas partial pressure, occurrence of H2S impurities, temperature) and considering different cells materials and microstructure (current collecting layer, diffusion barrier layer). The comparison of the different Nyquist plots, Bode plots and fitting results allows identifying the main electrochemical mechanisms occurring at the anode and the cathode and contributing to the polarization resistance of the cell. The main mechanisms associated to the anode were observed at ca. 0.1 Hz (fuel partial pressure) and ca. 600 Hz (anode charge transfer reaction). The anode contribution at low frequency (0.01-0.1 Hz) is particularly noticeable under methane internal steam reforming condition. Cathode contributions such as oxygen gas conversion and oxygen surface diffusion have been identified at ca. 0.02 Hz and 2-4 Hz, respectively. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.