NH4NO3 can form from intermediates present in low temperature (≤ 200 °C) DeNOx reactions. At 200°C, NH4NO3 is thermally stable and has been speculated as a possible poison in low temperature DeNOx catalysis. In this study, we investigated the ability of NO to reduce NH4NO3 to thermally less stable NH4NO2, which decomposes to N2 and H2O at ∼100°C, thus mitigating possible catalyst poisoning. Ground NH4NO3 was physically mixed with different substrates, then exposed to an NO flow at elevated temperature with FTIR monitoring of the effluent. The data show that reduction of neat NH4NO3 by NO takes place above its melting point (170°C), while on weakly acidic substrates reduction of NH4NO3 by NO takes place below 100°C. A possible mechanism for the acid catalyzed reduction of NH4NO3 by NO will be discussed, as well as implications for the effective temperatures for low temperature DeNOx catalysis.