Chemical degradation of anodes in solid oxide fuel cells (SOFCs) induced by phosphorous impurities has been investigated by thermochemical calculations, electrochemical characterization, and microstructural analysis. Thermochemical calculations indicate that Ni phosphides (NimPn) such as Ni5P2 are readily formed under SOFC operational conditions even in the case where the concentration of phosphorous impurities is as low as ppb level. Phosphorous impurities in fuels resulted in the formation of the secondary phase Ni phosphides during operation especially at an operating temperature similar to 1000 degrees C. The formation of Ni phosphides and change in anode microstructure were confirmed by using a field-emission-scanning electron microscope coupled with an energy dispersive X-ray analyzer, scanning transmission electron microscope, and X-ray diffraction, leading to the fatal cell performance degradation in the manner that anodic overpotential increased, current incorporation was obstructed, and internal fuel reforming reaction was deactivated. (c) 2010 The Electrochemical Society. [DOI: 10.1149/1.3489265] All rights reserved.