Three distinctly different characteristic responses of a nickel/zirconia solid oxide fuel cell (SOFC) anode to the presence of 0.5-5 ppm hydrogen selenide in synthetic coal gas were observed, depending on temperature, pH(2)Se, and especially on the extent of anodic polarization. The first level of response was characterized by a modest decrease in power density to a new steady state. Mostly observed at high temperatures, low pH(2)Se, and low anodic polarizations, this response was similar to the effects caused by H2S, only with slower onset and lower reversibility. Higher anodic polarization at a constant current triggered a second level of response characterized by oscillatory behavior involving performance loss followed by recovery. Oscillations ceased when the current density was lowered. A third level of response, irreversible cell failure, could be induced by the increase in anodic polarization, additionally favored by low temperature and high pH(2)Se. Post-test analyses of failed cells revealed the extensive microstructural changes including the appearance of nickel selenide and nickel oxide at the anode/electrolyte interface. From bulk thermochemical considerations, the formation of nickel selenides could not be expected. Local chemical conditions created at the active interface appear to be of overriding importance with respect to the extent of Ni interactions with H2Se in coal gas. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3505936] All rights reserved.