Anew optical microscope technique has been developed to investigate phase transitions in micrometer-sized droplets. This technique has been used to study the nucleation of ice from aqueous H2SO4 aerosols 0-35 wt % in composition in the temperature range from 273 to 170 K. The aerosols were produced with a nebulizer and were deposited on a quartz plate, which was coated with a hydrophobic silane monolayer to minimize the effects of heterogeneous nucleation. More than 1200 aerosol particles were monitored individually with the optical microscope, and their freezing temperatures and melting points were recorded. The observed freezing temperatures are lower than the ones from comparable aerosol studies reported in the literature, the differences in the freezing temperature being up to 30 K, especially for the more concentrated aerosols. No freezing was observed above 170 K for compositions greater than 27 wt %. A thermodynamic model has been used to apply the new freezing temperature data to the formation of clouds in the upper troposphere and lower stratosphere. 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 1.6 at 205 K. In addition, the formation of type II polar stratospheric clouds under volcanically perturbed conditions where H2SO4 is the main aqueous aerosol component at low temperatures is predicted to occur about 3 K below the ice frost point.