The deliquescence and efflorescence phase transitions of ammonium sulfate aerosols have been studied as a function of relative humidity (RH) over the temperature range from 234 K to 295 K. Polydisperse submicrometer ammonium sulfate particles produced by atomization were monitored in a temperature-controlled flow tube system using Fourier transform infrared spectroscopy. The relative humidity in the aerosol flow was controlled using a sulfuric acid bath conditioner and the addition of a known flow of dry nitrogen. The relative humidity was measured using a dew point hygrometer and infrared absorption features. The deliquescence transition was observed to be nearly independent of temperature, changing from 80% RH at 294.8 K to 82% RH at 258.0 K near the ice saturation line, in good agreement with previous results. The relative humidity at the efflorescence transition also increased slightly (32% to 39%) with decreasing temperature (294.8 K to 234.3 K). These results suggest that once a crystalline ammonium sulfate particle deliquesces, the droplet can exist as a metastable solution droplet over a broad region of temperature and water pressures under the conditions in the upper troposphere. The persistence of metastable ammonium sulfate solution droplets may have important implications for cirrus cloud formation and heterogeneous reaction rates in the upper troposphere. ¿ 1999 American Geophysical Union