The poisoning effects of various trace contaminants in the coal-derived syngas stream at ppm and sub-ppm level on the performance of Ni-YSZ/YSZ/LSM solid oxide fuel cells were studied at extended duration. The thermochemical nature of impurities such as PH3(g) and CH3Cl(g) in presence and absence of water steam was analyzed by a high temperature mass spectrometer. Only less than half of PH3(g) is hydrolyzed, and CH3Cl(g) also co-exist with HCl(g). After a certain duration of exposure, 1 ppm AsH3(g), 0.5 ppm PH3(g), and 2.5 ppm CH3Cl(g) all caused some degree of degradation to the power density at 750 degrees C. Whereas 1 ppm of H2S(g) resulted in immediate performance loss. The mechanisms of degradation are mainly divided into two categories: surface adsorption effect (for S and Cl) and bulk reaction effect (for As and P). The controversies regarding the poisoning effect and mechanism of S are also discussed with the aid of thermodynamic equilibrium composition calculation. (C) 2009 Elsevier B.V. All rights reserved.