Ice freezing events in aerosols composed of (NH4)(2)SO4/H2O particles are studied for 0 < x < 0.12, where x is the salt mole fraction composition. A segmented flow tube is employed to cool the aerosol and to adjust the aqueous particle composition in the first section, to freeze the aerosol in the second section, and to observe particle phase and composition with infrared (IR) spectroscopy in the third section. For the described experiments, three separate test aerosols are generated. The first test aeorsol is composed of aqueous particles, while the other two aerosols contain a mix of aqueous and crystalline species. The IR spectra of these three different aerosols are similar, and the same apparent mole fraction composition is indicated for each based upon the ratio of band intensities. Although they have similar apparent mole fraction compositions, the observed ice freezing characteristics of the aerosols differ markedly from one another. The first aerosol, which consists entirely of aqueous particles, shows the deepest apparent supercooling. The freezing data, consisting of ice mass fraction versus temperature, are inverted by a model simulation to obtain homogeneous nucleation rates (J) as a function of temperature (T) and composition (x). J(T, x) is provided for 0 < x < 0.12 and 5 < Log(10)J < 10, which is the range of J that can be probed with the current apparatus. At lower J values, which are associated with warmer temperatures, one particle in 10(6) nucleates ice homogeneously: the observed ice freezing event is then due to vapor-phase mass transport from the numerous surrounding supercooled aqueous particles to the few ice particles. In contrast, at higher J values, which are associated with cooler temperatures, one particle in 10 freezes by homogeneous nucleation.