Motivated by the roles which tropospheric aerosols play in cloud formation processes and heterogeneous chemistry, low-temperature phase transitions of submicron-sized (NH4)2SO4 aerosols have been directly observed using Fourier transform infrared extinction spectroscopy. In particular, deliquescence relative humidities have been measured down to 254 K and efflorescence points to 238 K. In agreement with previous studies, it was observed that solid particles deliquesced at the thermodynamic value, whereas efflorescence of aqueous aerosols did not occur until a substantially lower relative humidity was reached. By observing the formation of spectral features due to ice, the same apparatus was used to monitor freezing of supercooled, micron-sized aerosol particles over a composition range from 0 to 49 weight percent (wt%) (NH4)2SO4. The freezing of pure water aerosols occurred at 39 deg of supercooling, i.e., at a temperature of 234 K, in good agreement with the literature value. Ammonium sulfate aerosols exhibited significantly less than 39 deg of supercooling when referenced to the equilibrium freezing line. These results indicate that ammonium sulfate aerosols will promote cirrus cloud formation in the upper troposphere at higher temperatures and lower relative humidities than previously believed. ¿ 1999 American Geophysical Union