Ammonium nitrate is thermally stable below 250 degrees C and could potentially deactivate low temperature NOx reduction catalysts by blocking active sites. It is shown that NO reduces neat NH4NO3 above its 170 degrees C melting point, while acidic solids catalyze this reaction even at temperatures below 100 degrees C. NO2, a product of the reduction, can dimerize and then dissociate in molten NH4NO3 to NO+ NO3-, and may be stabilized within the melt as either an adduct or as HNO2 formed from the hydrolysis of NO+ or N2O4. The other product of reduction, NH4NO2, readily decomposes at <= 100 degrees C to N-2 and H2O, the desired end products of DeNO(x) catalysis. A mechanism for the acid catalyzed reduction of NH4NO3 by NO is proposed, with HNO3 as an intermediate. These findings indicate that the use of acidic catalysts or promoters in DeNO(x) systems could help mitigate catalyst deactivation at low operating temperatures (<150 degrees C). (C) 2008 Elsevier B.V. All rights reserved.