Ice nucleation in upper tropospheric aerosols is believed to be a key step in upper tropospheric cloud formation, and therefore knowledge of the conditions leading to ice formation in these aerosols is crucial. This paper addresses homogeneous nucleation of ice in (NH4)(2)SO4-H2O aerosols, a possible upper tropospheric aerosol. Two complementary techniques were employed in this study. First, differential scanning calorimetry was used to determine the temperature at which ice nucleates in emulsified solutions of ammonium sulfate and water. Second, optical microscopy was used to determine the temperature at which ice nucleates in individual ammonium sulfate-water particles. The results from these two techniques, which are in very good agreement, indicate that the freezing temperature of ammonium sulfate-water particles (approximately 10 mu m in size) ranges from 235 K for 0 wt % to 195 K for 40 wt %. These freezing temperatures correspond to saturation ratios with respect to ice ranging from 1.45 at 235 K to 1.68 at 195 K, which are similar to the saturation ratios required for homogeneous freezing of H2SO4-H2O and (NH4)HSO4-H2O particles. Based on these saturations, we conclude that both homogeneous and heterogeneous nucleation are important in upper tropospheric cloud formation.