To better understand atmospheric aerosol phase transitions under free tropospheric conditions, the behavior of ammonium sulfate (AS) particles mounted on a silicon substrate has been investigated between 229 and 295 K. Experiments were performed in a chamber where water partial pressure and particulate infrared spectra were monitored as the temperature was lowered. Deliquescence experiments at temperatures above the eutectic (254 K) are not new, however results defining deliquescence relative humidity (DRH) at temperatures below the eutectic have been lacking. At sub-eutectic temperatures the system is supersaturated with respect to ice, therefore both deliquescence and ice formation were possible. Within experimental error, deliquescence was observed at the thermodynamically predicted value both above and below the eutectic. The lack of a significant kinetic barrier to deliquescence implies that AS particles could form supercooled solution droplets at high relative humidities, and function as ice nuclei if these droplets homogeneously freeze. ¿ 2001 American Geophysical Union