The low-temperature phase behavior of ammonium sulfate ((NH4)2SO4) films has been studied using Fourier transform infrared (FTIR) spectroscopy. While the deliquescence of ammonium sulfate aerosols is well characterized at temperatures above the eutectic of ice and anhydrous ammonium sulfate at 254 K, much less is known about the phase properties at lower temperatures. In the present study, experiments were performed over the temperature range from 166 to 235 K. The apparatus used for this work was a thin-film, high-vacuum apparatus in which the condensed phase is monitored via FTIR spectroscopy and water pressure is monitored with an MKS baratron. Results of experiments performed at low relative humidity (RH) confirm the presence of a ferroelectric phase of ammonium sulfate at temperatures less than 216 ¿ 8 K. Results of experiments performed as a function of increasing RH demonstrate that a phase transition from crystalline (NH4)2SO4 to a metastable aqueous solution (hereafter referred to as deliquescence) occurs at temperatures below the eutectic. Specifically, at temperatures >203 K we observed deliquescence near 88 ¿ 8% RH. These results are in satisfactory agreement with extrapolated results from experiments performed at temperatures above the eutectic, as well as theory. In experiments performed at temperatures between 166 and 203 K, we sometimes observed deliquescence and sometimes observed direct deposition of ice from the vapor phase, possibly indicating selective heterogeneous nucleation. Ice nucleation prevents the relative humidity from rising to the level needed for deliquescence, thus explaining the variability in our low-temperature results. Possible implications of this work for cirrus cloud formation are also presented.