Low-temperature (T ≤ 200°C) NOx conversion is receiving increasing research attention due to continued potential reductions in regulated NOx emissions from diesel engines. At these temperatures, ammonium salts (e.g., ammonium nitrate, ammonium (bi)sulfate, etc.) can form as a result of interactions between NH3 and NOx or SOx, respectively. These formation of these salts can reduce the availability of NH3 for NOx conversion, block active catalyst sites, and result in the formation of N2O, a regulated Greenhouse Gas (GHG). In this study, we investigate the effect of hydrothermal aging on the formation and decomposition of ammonium nitrate on a state-of-the-art Cu/zeolite selective catalytic reduction (SCR) catalyst. Reactor-based constant-temperature ammonium nitrate formation and temperature programmed oxidation (TPO) and NO titration decomposition experiments are used to characterize the effect of hydrothermal aging from 600 to 950°C. NOx conversion, N2 adsorption (BET) surface area, and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) experiments are also conducted to correlate the morphological effects of hydrothermal aging with concomitant changes in ammonium nitrate chemistry. The insights provided herein support the diesel aftertreatment communities’ ongoing efforts to understand low-temperature chemical processes such as ammonium salt formation.