Though ammonium bisulfate is one of the most common of atmospheric hygroscopic aerosols, knowledge of its interaction with water has, until now, been extremely limited. This paper presents our observations on single isolated ammonium bisulfate aerosol particles, as they interact with water vapor at temperatures ranging from -40 to 30 degrees C. The complete phase diagram in the temperature/composition as well as the pressure/temperature domains has been derived here for the first time, mapping out equilibrium and metastable to stable phase transitions. A new low-temperature crystalline hydrate phase that might have an important role in processes occurring in the upper troposphere has been discovered.