Differential scanning calorimetry has been employed to investigate phase transitions in aqueous NH4HSO4 solutions 0 to 60 wt %. Bulk samples, 3 mu L in volume, have been used to determine the NH4HSO4 phase diagram down to 195 K. The results are in very good agreement with the ice, letovicite, and sulfuric acid tetrahydrate melting paints predicted by the thermodynamic model of Clegg et al. (J. Phys. Chem. A 1998, 102, 2137-2154). In addition, emulsified samples have been employed to investigate the kinetics of ice nucleation in micrometer-sized aqueous NH4HSO4 droplets. The observed ice freezing points range from 234 K for 0 wt % to 179 K for 35 wt %. A thermodynamic model has been used to apply these freezing point data to the formation of cirrus clouds. The results indicate that the homogeneous nucleation of ice particles in cirrus clouds requires saturation ratios with respect to ice ranging from about 1.5 at 230 K to about 1.7 at 200 K.