Poisoning effects by chlorine compounds, including Cl-2 and HCl, to typical electrolyte-supported solid oxide fuel cells (SOFCs) with Ni-scandia-stabilized zirconia cermet anodes have been evaluated. The degradation rate of cell voltage poisoned by 5 ppm Cl-2 was estimated to be ca. 3%/1000 h. Chlorine degradation rate increased with increasing Cl-2 concentration. Microstructural change associated with the formation of nickel nanoparticles on zirconia grains, probably via NiCl2(g) sublimation, was observed after 150 h poisoning tests, whereas a partial recovery of cell voltage by removing chlorine from the fuels indicates that the chlorine poisoning is partially reversible. Thermochemical calculations, microstructural analysis, and electrochemical characterizations have revealed that the poisoning phenomena for Ni-based cermet anodes caused by chlorine compounds can be explained by the mixed adsorption-type and sublimation-type poisoning mechanism. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2980521] All rights reserved.