Deposition of ammonium sulfates over catalysts is often observed during selective catalytic reduction (SCR) of NOx by NH3 in the presence of SO2 and H2O, especially at temperatures lower than 350 degrees C, which results in catalyst deactivation and equipment erosion. Activated coke was reported to be a support effective in reducing accumulation of ammonium sulfates on the catalyst surface. However, detailed information on stability of ammonium sulfates and the properties of coke affecting the stability have not been explored. This work studies decomposition behavior of NH4HSO4 deposited on different cokes in a TG-MS system. It shows that NH4HSO4 on coke decomposes in two steps: formation of NH3 and H2SO4 starting at about 170 degrees C and reduction of H2SO4 by coke to form SO2 at higher temperatures, regardless of the presence of NO. The decreased NH4HSO4 poisoning is attributed to reduction of H2SO4 because it frees the pores of coke. The BET surface area and pore volume of coke do not affect the reduction behavior of H2SO4 if only they are high enough; the oxygen and nitrogen containing functional groups in coke promote the reduction of H2SO4; the minerals in coke may interact with H2SO4 to form stable sulfates; V2O5 on coke and NO in the gas elevate the reduction temperature of H2SO4. (C) 2011 Elsevier B.V. All rights reserved.