Impedance spectroscopy and current-voltage measurements were used to study the synergistic effects of the coal gas contaminants As, P, S, and Cl at the parts per million level on the performance of the anode-supported Ni-yttria-stabilized zirconia (YSZ)/YSZ/lanthanum strontium manganese solid oxide fuel cells (SOFCs) at 750 degrees C. Three semicircles were differentiated in cell impedance spectra. The results indicate that the effects of the contaminants are synergistic, can be destructive or constructive, and are not a simple addition of individual contaminants. Adding H2S to As and P accelerated the performance degradation. The addition of H2S also increased the charge-transfer resistance, whereas the removal of H2S resulted in a substantial ohmic resistance increase. The presence of Cl could mitigate or prevent performance loss from an attack by As and P. Impedance spectra show that the presence of Cl species protected the electronic percolation and slowed the rate of the charge-transfer resistance increase. Therefore, the tolerance limit of SOFCs for each contaminant could be significantly increased with Cl species in the gas stream. It is speculated that Cl species could react with nickel phosphides and Ni-P alloy to form NiCl(g), resulting in the restructuring of the catalyst surface.