The formation rate and reaction rate of ammonium sulfate salts on V2O5/AC catalyst during selective catalytic reduction (SCR) of NO with NH3 at low temperatures were studied using elemental analysis, transient response, and TPR methods. In the presence of SO2 and H2O, ammonium sulfate salts deposit on the surface of the V2O5/AC catalyst, block the pores of the catalyst, and cause catalyst deactivation. Moreover, the deactivation rate increases with increasing H2O content in the flue gas. The deposition rate of the ammonium sulfate salts is governed by the rate difference between its formation and reaction with NO. In the presence of H2O, the formation rate of the ammonium sulfate salts is higher than that in the absence of H2O, and the reaction rate between the formed ammonium sulfate salts and NO is lower than that in the absence of H2O. SO2 does not affect the reaction rate of the ammonium sulfate salts with NO. FT-IR and XRD analysis show that the ammonium sulfate salts formed in the presence of SO2 and H2O on the catalyst surface at 250 degreesC is mainly (NH4)(3)H(SO4)(2). (C) 2003 Elsevier Science (USA). All rights reserved.