The two specific processes of hydrogen sulfide poisoning a solid oxide fuel cell (SOFC) were studied by impedance spectroscopy in galvanostatic mode at 800 degrees C. The polarization resistance increased by exposure to H2S within 30 min and then remained nearly stable in the subsequent aging test, accounting for an abrupt cell performance drop. A successive sluggish degradation was corresponding to the slow increase of the series resistance, which can be attributed to the contaminated nickel catalyst at the yttria-stabilized zirconia (Ni/YSZ) anode supported layer. The results indicated the establishment of a two-step mechanism of H2S poisoning an SOFC. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3526134] All rights reserved.