The impact of sulfur-poisoning on the electrochemistry of anode-supported solid oxide fuel cells (SOFCs) has been analyzed via electrochemical impedance spectroscopy. Therefore, single cells manufactured by Forschungszentrum Julich (FZJ) were operated with a reformate fuel. During operation, contents of H2S (0.1 and 0.5 ppm) were added to the fuel. A detailed analysis of the measured impedance spectra by the distribution of relaxation times (DRT) and a subsequent complex nonlinear least squares (CNLS) fit enabled a separated treatment of the resulting degradation of anodic and cathodic polarization processes. During the exposure to 0.5 ppm H2S, the polarization resistance increased by 200%, leveling off after 24 h. This strong increase has mainly been attributed to the decreased reaction rate of the electrochemical fuel oxidation. Additionally, a strongly decreased conversion rate of the water-gas shift reaction was determined from the impedance measurements. This decreased shift conversion rate results in a dramatically lowered utilization of the CO within the fuel. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.015206jes] All rights reserved.