Operating the SCR DeNO(x) reactor at temperatures below 200 degreesC results in a considerable saving in operating costs. Plant experience shows that on the catalysts in these second generation DeNO(x) plants, even for flue gases with SO2 concentration below 10 mg/m(3), over 1-2 years operating time sizeable quantities of ammonium sulfates accumulate. Ammonium sulfates deposited on V2O5-WO3/TiO2 catalysts react with NOx to nitrogen and sulfuric acid. Second-order rate constants of this reaction for temperatures of 170 degreesC have been derived. It could be shown that the sulfuric acid formed on the catalyst is displaced by water vapour and desorbs resulting in gas phase concentrations of up to 6.5 mg acid/m(3) flue gas. Plant equipment downstream of the ammonium sulfate containing low temperature DeNO(x) catalysts has to be protected against the corrosive action of the sulfuric acid in the flue gases leaving the DeNO(x) reactor.